Massive Stars (massive + star)

Distribution by Scientific Domains


Selected Abstracts


Massive stars exploding in a He-rich circumstellar medium , I. Type Ibn (SN 2006jc-like) events

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
A. Pastorello
ABSTRACT We present new spectroscopic and photometric data of the Type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal Type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf,Rayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf,Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a Type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous Type Ia event with some resemblance to SN 1991bg. [source]


Massive stars exploding in a He-rich circumstellar medium , II.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
The transitional case of SN 2005la
ABSTRACT We present photometric and spectroscopic data of the peculiar SN 2005la, an object which shows an optical light curve with some luminosity fluctuations and spectra with comparably strong narrow hydrogen and helium lines, probably of circumstellar nature. The increasing full width at half-maximum velocity of these lines is indicative of an acceleration of the circumstellar material. SN 2005la exhibits hybrid properties, sharing some similarities with both Type IIn supernovae and 2006jc-like (Type Ibn) events. We propose that the progenitor of SN 2005la was a very young Wolf,Rayet (WN-type) star which experienced mass ejection episodes shortly before core collapse. [source]


Masses and luminosities of O- and B-type stars and red supergiants

ASTRONOMISCHE NACHRICHTEN, Issue 4 2010
M.M. Hohle
Abstract Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O- and early B-type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi-colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O- and early B-type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground-based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ,20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Masters of the universe?

ASTRONOMY & GEOPHYSICS, Issue 4 2010
Matt Austin
Meeting report Massive stars make a profound impact on their surroundings, their galaxies and the evolution of the universe as a whole. Matt Austin and Dave Stock report on an RAS Discussion Meeting that considered current progress in understanding these complex stars. [source]


Massive stars exploding in a He-rich circumstellar medium , I. Type Ibn (SN 2006jc-like) events

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008
A. Pastorello
ABSTRACT We present new spectroscopic and photometric data of the Type Ibn supernovae 2006jc, 2000er and 2002ao. We discuss the general properties of this recently proposed supernova family, which also includes SN 1999cq. The early-time monitoring of SN 2000er traces the evolution of this class of objects during the first few days after the shock breakout. An overall similarity in the photometric and spectroscopic evolution is found among the members of this group, which would be unexpected if the energy in these core-collapse events was dominated by the interaction between supernova ejecta and circumstellar medium. Type Ibn supernovae appear to be rather normal Type Ib/c supernova explosions which occur within a He-rich circumstellar environment. SNe Ibn are therefore likely produced by the explosion of Wolf,Rayet progenitors still embedded in the He-rich material lost by the star in recent mass-loss episodes, which resemble known luminous blue variable eruptions. The evolved Wolf,Rayet star could either result from the evolution of a very massive star or be the more evolved member of a massive binary system. We also suggest that there are a number of arguments in favour of a Type Ibn classification for the historical SN 1885A (S-Andromedae), previously considered as an anomalous Type Ia event with some resemblance to SN 1991bg. [source]


The evolution of binary star clusters and the nature of NGC 2136/NGC 2137

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2007
S. F. Portegies Zwart
ABSTRACT We study the evolution of bound pairs of star clusters by means of direct N -body simulations. Our simulations include mass loss by stellar evolution. The initial conditions are selected to mimic the observed binary star clusters, NGC 2136 and 2137, in the Large Magellanic Cloud. Based on their rather old ages (,100 Myr), masses, sizes and projected separation, we conclude that the cluster pair must have been born with an initial separation of 15,20 pc. Clusters with a smaller initial separation tend to merge in ,60 Myr due to loss of angular momentum from escaping stars. Clusters with a larger initial separation tend to become even more widely separated due to mass loss from the evolving stellar populations. The early orbital evolution of a binary cluster is governed by mass loss from the evolving stellar population and by loss of angular momentum from escaping stars. Mass loss by stellar winds and supernovae explosions in the first ,30 Myr causes the binary to expand and the orbit to become eccentric. The initially less massive cluster expands more quickly than the binary separation increases, and is therefore bound to initiate mass transfer to the more massive cluster. This process is quite contrary to stellar binaries in which the more massive star tends to initiate mass transfer. Since mass transfer proceeds on a thermal time-scale from the less massive to the more massive cluster, this semidetached phase is quite stable, even in an eccentric orbit until the orbital separation reaches the gyration radius of the two clusters, at which point both clusters merge to one. [source]


Centrally condensed turbulent cores: massive stars or fragmentation?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005
Clare L. Dobbs
ABSTRACT We present numerical investigations into the formation of massive stars from turbulent cores of density structure ,,r,1.5. The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra P,k,4 and P,k,3.5, and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density (10,14 g cm,3). Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term. [source]


Dynamo-generated magnetic fields at the surface of a massive star

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
D. J. Mullan
ABSTRACT Spruit has shown that an astrophysical dynamo can operate in the non-convective material of a differentially rotating star as a result of a particular instability in the magnetic field (the Tayler instability). By assuming that the dynamo operates in a state of marginal instability, Spruit has obtained formulae which predict the equilibrium strengths of azimuthal and radial field components in terms of local physical quantities. Here, we apply Spruit's formulae to our previously published models of rotating massive stars in order to estimate Tayler dynamo field strengths. There are no free parameters in Spruit's formulae. In our models of 10- and 50-M, stars on the zero-age main sequence, we find internal azimuthal fields of up to 1 MG, and internal radial components of a few kG. Evolved models contain weaker fields. In order to obtain estimates of the field strength at the stellar surface, we examine the conditions under which the Tayler dynamo fields are subject to magnetic buoyancy. We find that conditions for Tayler instability overlap with those for buoyancy at intermediate to high magnetic latitudes. This suggests that fields emerge at the surface of a massive star between magnetic latitudes of about 45 and the poles. We attempt to estimate the strength of the field which emerges at the surface of a massive star. Although these estimates are very rough, we find that the surface field strengths overlap with values which have been reported recently for line-of-sight fields in several O and B stars. [source]


Accretion in stellar clusters and the collisional formation of massive stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Ian A. Bonnell
Abstract We present results from a numerical simulation of gas accretion in a cluster containing 1000 stars. The accretion forces the cluster to contract, leading to the development of a high-density core with a maximum density 105 times the mean stellar density. This density is sufficient for a significant number of stellar collisions to occur, resulting in the most massive stars being formed through a combination of gas accretion and stellar mergers. In the simulation, 19 mergers occur, generally where a binary is forced to merge owing to the interaction with another star in a small- N group. These small- N groups form owing to the self-gravity of the gas and constitute the highest-density regions in the cluster. Binary formation in these groups is common, occurring through dynamical three-body capture. The massive stars are thus generally in binary systems, which can be relatively wide. The self-gravity of the gas also forms significant structure in the vicinity of the cluster core, while continuing infall forms a circumbinary disc around the most massive star. This structure may be the source of collimation for the observed outflows from young massive stars. Finally, the resultant initial mass function from the combination of gas accretion and stellar mergers is indistinguishable from a Salpeter mass function. [source]


A study of the massive star-forming region M8 using images from the Spitzer Infrared Array Camera

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2010
Dewangan Lokesh Kumar
ABSTRACT We present photometry and images (3.6, 4.5, 5.8 and 8.0 ,m) from the Spitzer Infrared Array Camera (IRAC) of the star-forming region Messier 8 (M8). The IRAC photometry reveals ongoing star formation in the M8 complex, with 64 class 0/I and 168 class II sources identified in several locations in the vicinity of submm gas cores/clumps. Nearly 60 per cent of these young stellar objects (YSOs) occur in about seven small clusters. The spatial surface density of the clustered YSOs is determined to be about 10,20 YSOs pc,2. Fresh star formation by the process of ,collect and collapse' might have been triggered by the expanding H ii regions and winds from massive stars. IRAC ratio images are generated and studied in order to identify possible diagnostic emission regions in M8. The image of 4.5/8.0 ,m reveals a Br, counterpart of the optical Hourglass H ii region, while the ratio 8.0/4.5 ,m indicates PAH emission in a cavity-like structure to the east of the Hourglass. The ratio maps of 3.6/4.5, 5.8/4.5 and 8.0/4.5 ,m seem to identify PAH emission regions in the sharp ridges and filamentary structures seen east to west and north-east to south-west in the M8 complex. [source]


Can mass loss and overshooting prevent the excitation of g-modes in blue supergiants?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009
M. Godart
ABSTRACT Thanks to their past history on the main-sequence phase, supergiant massive stars develop a convective shell around the helium core. This intermediate convective zone (ICZ) plays an essential role in governing which g-modes are excited. Indeed, a strong radiative damping occurs in the high-density radiative core but the ICZ acts as a barrier preventing the propagation of some g-modes into the core. These g-modes can thus be excited in supergiant stars by the ,-mechanism in the superficial layers due to the opacity bump of iron, at log T= 5.2. However, massive stars are submitted to various complex phenomena such as rotation, magnetic fields, semiconvection, mass loss, overshooting. Each of these phenomena exerts a significant effect on the evolution and some of them could prevent the onset of the convective zone. We develop a numerical method which allows us to select the reflected, thus the potentially excited, modes only. We study different cases in order to show that mass loss and overshooting, in a large enough amount, reduce the extent of the ICZ and are unfavourable to the excitation of g-modes. [source]


Stellar contents and star formation in the young star cluster Be 59

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2008
A. K. Pandey
ABSTRACT We present UBV Ic CCD photometry of the young open cluster Be 59 with the aim to study the star formation scenario in the cluster. The radial extent of the cluster is found to be ,10 arcmin (2.9 pc). The interstellar extinction in the cluster region varies between E(B,V) , 1.4 to 1.8 mag. The ratio of total-to-selective extinction in the cluster region is estimated as 3.7 0.3. The distance of the cluster is found to be 1.00 0.05 kpc. Using near-infrared (NIR) colours and slitless spectroscopy, we have identified young stellar objects (YSOs) in the open cluster Be 59 region. The ages of these YSOs range between <1 and ,2 Myr, whereas the mean age of the massive stars in the cluster region is found to be ,2 Myr. There is evidence for second-generation star formation outside the boundary of the cluster, which may be triggered by massive stars in the cluster. The slope of the initial mass function, ,, in the mass range 2.5 < M/M,, 28 is found to be ,1.01 0.11 which is shallower than the Salpeter value (,1.35), whereas in the mass range 1.5 < M/M,, 2.5 the slope is almost flat. The slope of the K -band luminosity function is estimated as 0.27 0.02, which is smaller than the average value (,0.4) reported for young embedded clusters. Approximately 32 per cent of H, emission stars of Be 59 exhibit NIR excess indicating that inner discs of the T Tauri star (TTS) population have not dissipated. The Midcourse Space Experiment (MSX) and IRAS-HIRES images around the cluster region are also used to study the emission from unidentified infrared bands and to estimate the spatial distribution of optical depth of warm and cold interstellar dust. [source]


Colliding stellar wind models with non-equilibrium ionization: X-rays from WR 147

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2007
Svetozar A. Zhekov
ABSTRACT The effects of non-equilibrium ionization are explicitly taken into account in a numerical model which describes colliding stellar winds (CSW) in massive binary systems. This new model is used to analyse the most recent X-ray spectra of the WR+OB binary system WR 147. The basic result is that it can adequately reproduce the observed X-ray emission (spectral shape, observed flux) but some adjustment in the stellar wind parameters is required. Namely (i) the stellar wind velocities must be higher by a factor of 1.4,1.6 and (ii) the mass loss must be reduced by a factor of ,2. The reduction factor for the mass loss is well within the uncertainties for this parameter in massive stars, but given the fact that the orbital parameters (e.g. inclination angle and eccentricity) are not well constrained for WR 147, even smaller corrections to the mass loss might be sufficient. Only CSW models with non-equilibrium ionization and equal (or nearly equal) electron and ion post-shock temperature are successful. Therefore, the analysis of the X-ray spectra of WR 147 provides evidence that the CSW shocks in this object must be collisionless. [source]


A deep kinematic survey of planetary nebulae in the Andromeda galaxy using the Planetary Nebula Spectrograph

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006
H. R. Merrett
ABSTRACT We present a catalogue of positions, magnitudes and velocities for 3300 emission-line objects found by the Planetary Nebula Spectrograph in a survey of the Andromeda galaxy, M31. Of these objects, 2615 are found likely to be planetary nebulae (PNe) associated with M31. The survey area covers the whole of M31's disc out to a radius of . Beyond this radius, observations have been made along the major and minor axes, and the Northern Spur and Southern Stream regions. The calibrated data have been checked for internal consistency and compared with other catalogues. With the exception of the very central, high surface brightness region of M31, this survey is complete to a magnitude limit of m5007, 23.75, 3.5 mag into the PN luminosity function. We have identified emission-line objects associated with M31's satellites and other background galaxies. We have examined the data from the region tentatively identified as a new satellite galaxy, Andromeda VIII, comparing it to data in the other quadrants of the galaxy. We find that the PNe in this region have velocities that appear to be consistent with membership of M31 itself. The luminosity function of the surveyed PNe is well matched to the usual smooth monotonic function. The only significant spatial variation in the luminosity function occurs in the vicinity of M31's molecular ring, where the luminosities of PNe on the near side of the galaxy are systematically ,0.2 mag fainter than those on the far side. This difference can be explained naturally by a modest amount of obscuration by the ring. The absence of any difference in luminosity function between bulge and disc suggests that the sample of PNe is not strongly populated by objects whose progenitors are more massive stars. This conclusion is reinforced by the excellent agreement between the number counts of PNe and the R -band light. The number counts of kinematically selected PNe also allow us to probe the stellar distribution in M31 down to very faint limits. There is no indication of a cut-off in M31's disc out to beyond four scalelengths, and no signs of a spheroidal halo population in excess of the bulge out to 10 effective bulge radii. We have also carried out a preliminary analysis of the kinematics of the surveyed PNe. The mean streaming velocity of the M31 disc PNe is found to show a significant asymmetric drift out to large radii. Their velocity dispersion, although initially declining with radius, flattens out to a constant value in the outer parts of the galaxy. There are no indications that the disc velocity dispersion varies with PN luminosity, once again implying that the progenitors of PNe of all magnitudes form a relatively homogeneous old population. The dispersion profile and asymmetric drift results are shown to be mutually consistent, but require that the disc flares with radius if the shape of its velocity ellipsoid remains invariant. [source]


Triggered star formation in bright-rimmed clouds: the Eagle nebula revisited

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2006
J. Miao
ABSTRACT A three-dimensional smoothed particle hydrodynamics model has been extended to study the radiation-driven implosion effect of massive stars on the dynamical evolution of surrounding molecular clouds. The new elements in the upgraded code are the inclusion of Lyman continuum in the incident radiation flux and the treatment of hydrogen ionization process; the introduction of ionization heating and recombination cooling effects; and the addition of a proper description of the magnetic and turbulent pressures to the internal pressure of the molecular cloud. This extended code not only provides a realistic model to trace the dynamical evolution of a molecular cloud, but also can be used to model the kinematics of the ionization and shock fronts and the photoevaporating gas surrounding the molecular cloud, which the previous code is unable to handle. The application of this newly developed model to the structure of the middle Eagle nebula finger suggests that the shock induced by the ionizing radiation at the front side of the head precedes an ionization front moving towards the centre of the core, and that the core at the fingertip is at a transition stage evolving toward a state of induced star formation. The dynamical evolution of the velocity field of the simulated cloud structure is discussed to illustrate the role of the self-gravity and the different cloud morphologies which appear at different stages in the evolutionary process of the cloud. The motion of the ionization front and the evaporating gas are also investigated. The modelled gas evaporation rate is consistent with that of other current models and the density, temperature and chemical profiles are in agreement with the observed values. The relative lifetimes of different simulated cloud morphologies suggest a possible answer to the question of why more bright-rimmed clouds are observed to possess a flat-core than an elongated-core morphology. [source]


Erratum: A census of the Carina Nebula , I. Cumulative energy input from massive stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006
Nathan Smith
No abstract is available for this article. [source]


Centrally condensed turbulent cores: massive stars or fragmentation?

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2005
Clare L. Dobbs
ABSTRACT We present numerical investigations into the formation of massive stars from turbulent cores of density structure ,,r,1.5. The results of five hydrodynamical simulations are described, following the collapse of the core, fragmentation and the formation of small clusters of protostars. We generate two different initial turbulent velocity fields corresponding to power-law spectra P,k,4 and P,k,3.5, and we apply two different initial core radii. Calculations are included for both completely isothermal collapse, and a non-isothermal equation of state above a critical density (10,14 g cm,3). Our calculations reveal the preference of fragmentation over monolithic star formation in turbulent cores. Fragmentation was prevalent in all the isothermal cases. Although disc fragmentation was largely suppressed in the non-isothermal runs due to the small dynamic range between the initial density and the critical density, our results show that some fragmentation still persisted. This is inconsistent with previous suggestions that turbulent cores result in the formation of a single massive star. We conclude that turbulence cannot be measured as an isotropic pressure term. [source]


Dynamo-generated magnetic fields at the surface of a massive star

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2005
D. J. Mullan
ABSTRACT Spruit has shown that an astrophysical dynamo can operate in the non-convective material of a differentially rotating star as a result of a particular instability in the magnetic field (the Tayler instability). By assuming that the dynamo operates in a state of marginal instability, Spruit has obtained formulae which predict the equilibrium strengths of azimuthal and radial field components in terms of local physical quantities. Here, we apply Spruit's formulae to our previously published models of rotating massive stars in order to estimate Tayler dynamo field strengths. There are no free parameters in Spruit's formulae. In our models of 10- and 50-M, stars on the zero-age main sequence, we find internal azimuthal fields of up to 1 MG, and internal radial components of a few kG. Evolved models contain weaker fields. In order to obtain estimates of the field strength at the stellar surface, we examine the conditions under which the Tayler dynamo fields are subject to magnetic buoyancy. We find that conditions for Tayler instability overlap with those for buoyancy at intermediate to high magnetic latitudes. This suggests that fields emerge at the surface of a massive star between magnetic latitudes of about 45 and the poles. We attempt to estimate the strength of the field which emerges at the surface of a massive star. Although these estimates are very rough, we find that the surface field strengths overlap with values which have been reported recently for line-of-sight fields in several O and B stars. [source]


Surface abundances of light elements for a large sample of early B-type stars , III.

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
An analysis of helium lines in spectra of 102 stars
ABSTRACT Non-local thermodynamic equilibrium analysis of He i lines in spectra of 102 B stars is implemented in order to derive the helium abundance He/H, the microturbulent parameter Vt and the projected rotation velocity v sin i. A simultaneous determination of He/H and Vt for the stars is effected by analysing equivalent widths of the 4471- and 4922- lines primarily as indicators of He/H and the 4713-, 5016-, 5876- and 6678- lines primarily as indicators of Vt. The rotation velocities v sin i are found from profiles of the same lines. It is shown that, when Vt > 7 km s,1, the Vt(He i) values determined from He i lines are systematically overestimated as compared with the Vt(O ii, N ii) values derived from O ii and N ii lines. This discrepancy is especially appreciable for hot evolved B giants with Vt(He i) = 16,23 km s,1 and may indicate a failure of classical model atmospheres to represent the strong He i lines for these stars. Two programme stars, HR 1512 and 7651, are found to be helium-weak stars. The remaining 100 stars are divided into three groups according to their masses M. The microturbulent parameter Vt(He i) is low for all stars of group A (M= 4.1,6.9 M,) and for all stars with the relative ages t/tMS < 0.8 of group B (M= 7.0,11.2 M,). Their Vt(He i) values are within the 0 to 5 km s,1 range, as a rule; the mean value is Vt= 1.7 km s,1. Only evolved giants of group B, which are close to the termination of the main-sequence (MS) evolutionary phase (t/tMS > 0.8), show Vt(He i) up to 11 km s,1. The helium abundance He/H is correlated with the relative age t/tMS in both groups; the averaged He/H enhancement during the MS phase is 26 per cent. For group C, containing the most massive stars (M= 12.4,18.8 M,), the Vt(He i) values display a correlation with t/tMS, varying from 4 to 23 km s,1. The He/H determination for hot evolved B giants of the group with Vt(He i) > 15 km s,1 depends on a choice between the Vt(He i) and Vt(O ii, N ii) scales. The mean He/H enrichment by 67 per cent during the MS phase is found, if the abundances He/H are based on the Vt(O ii, N ii) scale; however, two evolved giants with especially high v sin i, HR 7446 and 7993, show the He/H enhancement by about a factor of 2.5. When using the same Vt scale, we found a trend of He/H with projected rotational velocities v sin i; a large dispersion for v sin i > 150 km s,1 can result from differences in masses M. A comparison with the stellar model computations with rotationally induced mixing shows that the observed helium enrichment during the MS phase can be explained by rotation with initial velocities 250,400 km s,1. The He/H distribution on M and v sin i based on the Vt(O ii, N ii) scale seems to be in better agreement with the theory than one based on the Vt(He i) scale. The mean value He/H = 0.10 derived for stars in the zero age main sequence (ZAMS) vicinity can be adopted as the typical initial helium abundance for early B stars in the solar neighbourhood. [source]


Ultraluminous X-ray sources and star formation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004
A. R. King
ABSTRACT Chandra observations of the Cartwheel galaxy reveal a population of ultraluminous X-ray sources (ULXs) with lifetimes , 107 yr associated with a spreading wave of star formation which began some 3 108 yr ago. A population of high-mass X-ray binaries provides a simple model: donor stars of initial masses M2, 15 M, transfer mass on their thermal time-scales to black holes of masses M1, 10 M,. For alternative explanations of the Cartwheel ULX population in terms of intermediate-mass black holes (IMBHs) accreting from massive stars, the inferred production rate ,10,6 yr,1 implies at least 300 IMBHs, and more probably 3 104, within the star-forming ring. These estimates are increased by factors of ,,1 if the efficiency , with which IMBHs find companions of ,15 M, within 107 yr is <1. Current models of IMBH production would require a very large mass (,1010 M,) of stars to have formed new clusters. Further, the accretion efficiency must be low (, 6 10,3) for IMBH binaries, suggesting super-Eddington accretion, even though intermediate black hole masses are invoked with the purpose of avoiding it. These arguments suggest either that, to make a ULX, an IMBH must accrete from some as yet unknown non-stellar mass reservoir with very specific properties, or that most if not all ULXs in star-forming galaxies are high-mass X-ray binaries. [source]


Infrared [Fe ii] emission in the circumstellar nebulae of luminous blue variables

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Nathan Smith
Abstract After a serendipitous discovery of bright [Fe ii],16435 emission in nebulae around , Carinae and P Cygni, infrared spectra of other luminous blue variables (LBV) and LBV candidates were obtained. Bright infrared [Fe ii] emission appears to be a common property among LBVs with prominent nebulae; this is an interesting discovery because strong [Fe ii],16435 is typically seen in shock-excited objects like supernova remnants and outflows from newly formed massive stars, as well as in active galactic nuclei (AGN), where the excitation mechanism is uncertain. This paper presents spectra in the H-band (1.5 to 1.75 ,m) for the central stars and nebulae of , Car, AG Car, P Cyg, Wra 751, HR Car, HD 168625, HD 160529, R 127 and S Doradus. Seven of nine targets show bright [Fe ii],16435 in their nebulae, while it is absent in all central stars except the LBV candidate Wra 751. The two objects (S Dor and HD 160529) without prominent [Fe ii],16435 are not yet known to have nebulae detected in optical images, and both lack bright thermal infrared emission from dust. The possible excitation mechanisms for this line and the implications of its discovery in LBV nebulae are discussed; there are good reasons to expect shock excitation in some objects, but other mechanisms cannot be ruled out. [source]


Accretion in stellar clusters and the collisional formation of massive stars

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2002
Ian A. Bonnell
Abstract We present results from a numerical simulation of gas accretion in a cluster containing 1000 stars. The accretion forces the cluster to contract, leading to the development of a high-density core with a maximum density 105 times the mean stellar density. This density is sufficient for a significant number of stellar collisions to occur, resulting in the most massive stars being formed through a combination of gas accretion and stellar mergers. In the simulation, 19 mergers occur, generally where a binary is forced to merge owing to the interaction with another star in a small- N group. These small- N groups form owing to the self-gravity of the gas and constitute the highest-density regions in the cluster. Binary formation in these groups is common, occurring through dynamical three-body capture. The massive stars are thus generally in binary systems, which can be relatively wide. The self-gravity of the gas also forms significant structure in the vicinity of the cluster core, while continuing infall forms a circumbinary disc around the most massive star. This structure may be the source of collimation for the observed outflows from young massive stars. Finally, the resultant initial mass function from the combination of gas accretion and stellar mergers is indistinguishable from a Salpeter mass function. [source]


Winds from massive stars: implications for the afterglows of , -ray bursts

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2001
Enrico Ramirez-Ruiz
Recent observations suggest that long-duration , -ray bursts (GRBs) and their afterglows are produced by highly relativistic jets emitted in core-collapse explosions. The pre-explosive ambient medium provides a natural test for the most likely progenitors of GRBs. Those stars that shed their envelopes most readily have short jet crossing times and are more likely to produce a GRB. We construct a simple computational scheme to explore the expected contribution of the presupernova ejecta of single Wolf,Rayet (WR) stars to the circumstellar environment. Using detailed stellar tracks for the evolution of massive stars, we discuss the effects that the initial main-sequence mass, metallicity, rotation and membership in a binary system have on the ambient medium. We extend the theory of GRB afterglows in winds to consider the effect of the relativistic fireball propagating through the WR ejecta. Specific predictions are made for the interaction of the relativistic blast wave with the density bumps that arise when the progenitor star rapidly loses a large fraction of its initial mass or when the ejected wind interacts with the external medium and decelerates. A re-brightening of the afterglow with a spectrum redder than the typical synchrotron spectrum (as seen in GRB 970508, GRB 980326 and GRB 000911) is predicted. We also calculate the luminosity of the reflected echo that arises when circumstellar material Compton-scatters the prompt radiation, and examine the spectral signatures expected from the interaction of the GRB afterglow with the ejected medium. [source]


Stellar dynamical evidence against a cold disc origin for stars in the Galactic Centre

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2008
Jorge Cuadra
ABSTRACT Observations of massive stars within the central parsec of the Galaxy show that, while most stars orbit within a well-defined disc, a significant fraction have large eccentricities and/or inclinations with respect to the disc plane. Here, we investigate whether this dynamically hot component could have arisen via scattering from an initially cold disc , the expected initial condition if the stars formed from the fragmentation of an accretion disc. Using N -body methods, we evolve a variety of flat, cold, stellar systems, and study the effects of initial disc eccentricity, primordial binaries, very massive stars and intermediate mass black holes. We find, consistent with previous results, that a circular disc does not become eccentric enough unless there is a significant population of undetected 100,1000 M, objects. However, since fragmentation of an eccentric disc can readily yield eccentric stellar orbits, the strongest constraints come from inclinations. We show that none of our initial conditions yields the observed large inclinations, regardless of the initial disc eccentricity or the presence of massive objects. These results imply that the orbits of the young massive stars in the Galactic Centre are largely primordial, and that the stars are unlikely to have formed as a dynamically cold disc. [source]


Masses and luminosities of O- and B-type stars and red supergiants

ASTRONOMISCHE NACHRICHTEN, Issue 4 2010
M.M. Hohle
Abstract Massive stars are of interest as progenitors of supernovae, i.e. neutron stars and black holes, which can be sources of gravitational waves. Recent population synthesis models can predict neutron star and gravitational wave observations but deal with a fixed supernova rate or an assumed initial mass function for the population of massive stars. Here we investigate those massive stars, which are supernova progenitors, i.e. with O- and early B-type stars, and also all supergiants within 3 kpc. We restrict our sample to those massive stars detected both in 2MASS and observed by Hipparcos, i.e. only those stars with parallax and precise photometry. To determine the luminosities we calculated the extinctions from published multi-colour photometry, spectral types, luminosity class, all corrected for multiplicity and recently revised Hipparcos distances. We use luminosities and temperatures to estimate the masses and ages of these stars using different models from different authors. Having estimated the luminosities of all our stars within 3 kpc, in particular for all O- and early B-type stars, we have determined the median and mean luminosities for all spectral types for luminosity classes I, III, and V. Our luminosity values for supergiants deviate from earlier results: Previous work generally overestimates distances and luminosities compared to our data, this is likely due to Hipparcos parallaxes (generally more accurate and larger than previous ground-based data) and the fact that many massive stars have recently been resolved into multiples of lower masses and luminosities. From luminosities and effective temperatures we derived masses and ages using mass tracks and isochrones from different authors. From masses and ages we estimated lifetimes and derived a lower limit for the supernova rate of ,20 events/Myr averaged over the next 10 Myr within 600 pc from the sun. These data are then used to search for areas in the sky with higher likelihood for a supernova or gravitational wave event (like OB associations) ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


A uvby, photometric analysis of the Monoceros star-forming field

ASTRONOMISCHE NACHRICHTEN, Issue 4 2010
N. Kaltcheva
Abstract This investigation presents a new analysis of the spatial distribution of the bright early-type stars in the field of Northern Monoceros. A database of all O,B9 stars with available uvby, photometry is collated and a homogeneous distance scale is established for the clusters and layers of field stars. We provide revised distances for NGC 2264 and NGC 2244 of 83338 (s.e.) pc and 158560 (s.e.) pc, respectively. We present arguments that there might be substructures in the clusters projected along the line of sight. According to the present sample the classical Mon OB2 association at 1.6 kpc is represented by a relatively compact group at 1.26 kpc in the vicinity of NGC 2244 and a layer of massive stars located between 1.5 and 3 kpc ( 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Metal enrichment of the intracluster medium: SN-driven galactic winds

ASTRONOMISCHE NACHRICHTEN, Issue 9-10 2009
V. Baumgartner
Abstract We investigate the role of supernova (SN)-driven galactic winds in the chemical enrichment of the intracluster medium (ICM). Such outflows on galactic scales have their origin in huge star forming regions and expel metal enriched material out ofthe galaxies into their surroundings as observed, for example, in the nearby starburst galaxy NGC 253. As massive stars in OB-associations explode sequentially, shock waves are driven into the interstellar medium (ISM) of a galaxy and merge, forming a superbubble (SB). These SBs expand in a direction perpendicular to the disk plane following the density gradient of the ISM. We use the 2D analytical approximation by Kompaneets (1960) to model the expansion of SBs in an exponentially stratified ISM. This is modified in order to describe the sequence of SN-explosions as a time-dependent process taking into account the main-sequence life-time of the SN-progenitors and using an initial mass function to get the number of massive stars per mass interval. The evolution of the bubble in space and time is calculated analytically, from which the onset of Rayleigh-Taylor instabilities in the shell can be determined. In its further evolution, the shell will break up and high-metallicity gas will be ejected into the halo ofthe galaxy and even into the ICM. We derive the number of stars needed for blow-out depending on the scale height and density ofthe ambient medium, as well as the fraction of alpha- and iron peak elements contained in the hot gas. Finally, the amount of metals injected by Milky Way-type galaxies to the ICM is calculated confirming the importance ofthis enrichment process ( 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


X-ray spectroscopy of early-type stars: The present and the future

ASTRONOMISCHE NACHRICHTEN, Issue 2 2008
G. Rauw
Abstract XMM-Newton and Chandra have boosted our knowledge about the X-ray emission of early-type stars (spectral types OB and Wolf-Rayet). However, there are still a number of open questions that need to be addressed in order to fully understand the X-ray spectra of these objects. Many of these issues require high-resolution spectroscopy or monitoring of a sample of massive stars. Given the moderate X-ray brightness of these targets, rather long exposure times are needed to achieve these goals. In this contribution, we review our current knowledge in this field and present some hot topics that could ideally be addressed with XMM-Newton over the next decade. ( 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Evolution of magnetic fields in stars across the upper main sequence: II.

ASTRONOMISCHE NACHRICHTEN, Issue 6 2007
Observed distribution of the magnetic field geometry
Abstract We re-discuss the evolutionary state of upper main sequence magnetic stars using a sample of Ap and Bp stars with accurate Hipparcos parallaxes and definitely determined longitudinal magnetic fields. We confirm our previous results obtained from the study of Ap and Bp stars with accurate measurements of the mean magnetic field modulus and mean quadratic magnetic fields that magnetic stars of mass M < 3 M, are concentrated towards the centre of the main-sequence band. In contrast, stars with masses M > 3 M, seem to be concentrated closer to the ZAMS. The study of a few known members of nearby open clusters with accurate Hipparcos parallaxes confirms these conclusions. Stronger magnetic fields tend to be found in hotter, younger and more massive stars, as well as in stars with shorter rotation periods. The longest rotation periods are found only in stars which spent already more than 40% of their main sequence life, in the mass domain between 1.8 and 3 M, and with log g values ranging from 3.80 to 4.13. No evidence is found for any loss of angular momentum during the main-sequence life. The magnetic flux remains constant over the stellar life time on the main sequence. An excess of stars with large obliquities , is detected in both higher and lower mass stars. It is quite possible that the angle , becomes close to 0. in slower rotating stars of mass M > 3 M, too, analog to the behaviour of angles , in slowly rotating stars of M < 3 M,. The obliquity angle distribution as inferred from the distribution of r -values appears random at the time magnetic stars become observable on the H-R diagram. After quite a short time spent on the main sequence, the obliquity angle , tends to reach values close to either 90. or 0. for M < 3 M,. The evolution of the obliquity angle , seems to be somewhat different for low and high mass stars. While we find a strong hint for an increase of , with the elapsed time on the main sequence for stars with M > 3 M,, no similar trend is found for stars with M < 3 M,. However, the predominance of high values of , at advanced ages in these stars is notable. As the physics governing the processes taking place in magnetised atmospheres remains poorly understood, magnetic field properties have to be considered in the framework of dynamo or fossil field theories. ( 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


Symbolic analytical solutions for the abundances differential equations of the Helium burning phase

ASTRONOMISCHE NACHRICHTEN, Issue 5 2003
M.I. Nouh
Abstract In this paper, a literal analytical solution is developed for the abundances differential equations of the helium burning phase in hot massive stars. The abundance for each of the basic elements 4He,12C,16O and 20Ne is obtained as a recurrent power series in time, which facilitates its symbolic and numerical evaluations. Numerical comparison between the present solution and the numerical integration of the differential equations for the abundances show good agreement. [source]