Central Star (central + star)

Distribution by Scientific Domains


Selected Abstracts


The role of thermodynamics in disc fragmentation

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
Dimitris Stamatellos
ABSTRACT Thermodynamics play an important role in determining the way a protostellar disc fragments to form planets, brown dwarfs and low-mass stars. We explore the effect that different treatments of radiative transfer have in simulations of fragmenting discs. Three prescriptions for the radiative transfer are used: (i) the diffusion approximation of Stamatellos et al.; (ii) the barotropic equation of state (EOS) of Goodwin et al. and (iii) the barotropic EOS of Bate et al. The barotropic approximations capture the general evolution of the density and temperature at the centre of each proto-fragment but (i) they do not make any adjustments for particular circumstances of a proto-fragment forming in the disc and (ii) they do not take into account thermal inertia effects that are important for fast-forming proto-fragments in the outer disc region. As a result, the number of fragments formed in the disc and their properties are different, when a barotropic EOS is used. This is important not only for disc studies but also for simulations of collapsing turbulent clouds, as in many cases in such simulations stars form with discs that subsequently fragment. We also examine the difference in the way proto-fragments condense out in the disc at different distances from the central star using the diffusion approximation and following the collapse of each proto-fragment until the formation of the second core (,, 10,3 g cm,3). We find that proto-fragments forming closer to the central star tend to form earlier and evolve faster from the first to the second core than proto-fragments forming in the outer disc region. The former have a large pool of material in the inner disc region that they can accrete from and grow in mass. The latter accrete more slowly and they are hotter because they generally form in a quick abrupt event. [source]


Multiple clump structures within photoionized regions

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009
A. C. Raga
ABSTRACT We present 3D simulations of a system of four neutral clumps embedded in a photoionized region. In this system, we have three small clumps which partially shield a single, larger clump from the stellar ionizing photons. This flow evolves to form a neutral structure with a main body and three neutral ,columns' pointing towards the central star. Qualitatively, similar structures are seen in the ,Finger' of the Carina Nebula. [source]


Near-infrared polarimetry and modelling of the dusty young planetary nebula IRAS 19306+1407

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2007
K. T. E. Lowe
ABSTRACT We present near-infrared polarimetric images of the dusty circumstellar envelope (CSE) of IRAS 19306+1407, acquired at the United Kingdom Infrared Telescope (UKIRT) using the UKIRT 1,5 ,m Imager Spectrometer (UIST) in conjunction with the half-waveplate module IRPOL2. We present additional 450- and 850-,m photometry data obtained with the Submillimetre Common-User Bolometer Array (SCUBA) at the James Clerk Maxwell Telescope (JCMT), as well as archived Hubble Space Telescope (HST) F606W - and F814W -filter images. The CSE structure in polarized flux at J and K bands shows an elongation north of north-east and south of south-west with two bright scattering shoulders north-west and south-east. These features are not perpendicular to each other and could signify a recent ,twist' in the outflow axis. We model the CSE using an axisymmetric light scattering (als) code to investigate the polarization produced by the CSE, and an axisymmetric radiation transport (dart) code to fit the spectral energy distribution. A good fit was achieved with the als and dart models using silicate grains, 0.1,0.4 ,m with a power-law size distribution of a,3.5, and an axisymmetric shell geometry with an equator-to-pole ratio of 7:1. The spectral type of the central star is determined to be B1i supporting previous suggestions that the object is an early planetary nebula. We have constrained the CSE and interstellar extinction as 2.0 and 4.2 mag, respectively, and have estimated a distance of 2.7 kpc. At this distance, the stellar luminosity is ,4500 L, and the mass of the CSE is ,0.2 M,. We also determine that the mass loss lasted for ,5300 yr with a mass-loss rate of ,3.4 10,5 M, yr,1. [source]


Evolution of the 3.3-,m emission feature in the Red Rectangle

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2003
In-Ok Song
ABSTRACT Medium-resolution infrared spectroscopic observations of the biconical Red Rectangle nebula in the region of the 3.3-,m unidentified infrared (UIR) emission band are reported. The data were recorded at UKIRT using CGS4 and have allowed the peak wavelength, width, profile and intensity of the 3.3-,m feature to be investigated as a function of offset from the central star HD 44179. Analysis of the profile of the feature along the north-western bicone interface shows an evolution from Type 2 to Type 1 in the classification of Tokunaga et al. The 3.3-,m band recorded on-star shows a close fit to a Lorentzian profile, possibly suggesting a single class of carriers. Subtraction of the Lorentzian fit to the on-star spectrum from the spectrum at each offset reveals a new ,3.28'-,m emission feature that grows in intensity relative to the main 3.3-,m band as a function of distance from the central star. The 3.28-,m emission band at large offset appears to correspond well in wavelength and FWHM with an absorption feature seen towards the Galactic Centre. [source]


Reversing type II migration: resonance trapping of a lighter giant protoplanet

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2001
F. Masset
We present a mechanism related to the migration of giant protoplanets embedded in a protoplanetary disc whereby a giant protoplanet is caught up, before having migrated all the way to the central star, by a lighter outer giant protoplanet. This outer protoplanet may get captured into the 2:3 resonance with the more massive one, in which case the gaps that the two planets open in the disc overlap. Two effects arise, namely a squared mass-weighted torque imbalance and an increased mass flow through the overlapping gaps from the outer disc to the inner disc, which both play in favour of an outwards migration. Indeed, under the conditions presented here, which describe the evolution of a pair of protoplanets respectively Jupiter- and Saturn-sized, the migration is reversed, while the semimajor axis ratio of the planets is constant and the eccentricities are confined to small values by the disc material. The long-term behaviour of the system is briefly discussed, and could account for the high eccentricities observed for the extrasolar planets with semimajor axis [source]


Stability of fictitious Trojan planets in extrasolar systems

ASTRONOMISCHE NACHRICHTEN, Issue 8 2007
R. Schwarz
Abstract Our work deals with the dynamical possibility that in extrasolar planetary systems a terrestrial planet may have stable orbits in a 1:1 mean motion resonance with a Jovian like planet. We studied the motion of fictitious Trojans around the Lagrangian points L4/L5 and checked the stability and/or chaoticity of their motion with the aid of the Lyapunov Indicators and the maximum eccentricity. The computations were carried out using the dynamical model of the elliptic restricted three-body problem that consists of a central star, a gas giant moving in the habitable zone, and a massless terrestrial planet. We found 3 new systems where the gas giant lies in the habitable zone, namely HD99109, HD101930, and HD33564. Additionally we investigated all known extrasolar planetary systems where the giant planet lies partly or fully in the habitable zone. The results show that the orbits around the Lagrangian points L4/L5 of all investigated systems are stable for long times (107 revolutions). ( 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]


The haloes of planetary nebulae in the mid-infrared: evidence for interaction with the interstellar medium

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009
G. Ramos-Larios
ABSTRACT The motion of planetary nebulae through the interstellar medium (ISM) is thought to lead to a variety of observational consequences, including the formation of bright rims, deformation and fragmentation of the shells, and a shift of the central stars away from the geometric centres of the envelopes. These and other characteristics have been noted through imaging in the visual wavelength regime. We report further observations of such shells taken in the mid-infrared (MIR), acquired through programmes of Infrared Array Camera imaging undertaken using the SpitzerSpace Telescope. NGC 2440 and NGC 6629 are shown to possess likely interacting haloes, together with ram-pressure-stripped material to one side of their shells. Similarly, the outer haloes of NGC 3242 and NGC 6772 appear to have been fragmented through Rayleigh,Taylor (RT) instabilities, leading to a possible flow of ISM material towards the inner portions of their envelopes. If this interpretation is correct, then it would suggest that NGC 3242 is moving towards the NE, a suggestion which is also supported through the presence of a 60 ,m tail extending in the opposite direction, and curved bands of H, emission in the direction of motion , components which may arise through RT instabilities in the magnetized ISM. NGC 2438 possesses strong scalloping at the outer limits of its asymptotic giant branch (AGB) halo, probably reflecting RT instabilities at the nebular/ISM interface We also note that the interior structure of the source has been interpreted in terms of a recombining shell, a hypothesis which may not be consistent with the central star luminosities. Finally, we point out that two of the rims (and likely shock interfaces) appear to have a distinct signature in the MIR, whereby relative levels of 8.0 ,m emission are reduced. This may imply that the grain emission agents are depleted in the post-shock AGB regimes. [source]


Using long-term transit timing to detect terrestrial planets

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2007
Jeremy S. Heyl
ABSTRACT We propose that the presence of additional planets in extrasolar planetary systems can be detected by long-term transit timing studies. If a transiting planet is on an eccentric orbit then the presence of another planet causes a secular advance of the transiting planet's pericentre over and above the effect of general relativity. Although this secular effect is impractical to detect over a small number of orbits, it causes long-term differences when future transits occur, much like the long-term decay observed in pulsars. Measuring this transit-timing delay would thus allow the detection of either one or more additional planets in the system or the first measurements of non-zero oblateness (J2) of the central stars. [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]