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Magnitude Diagrams (magnitude + diagram)
Selected AbstractsThe incidence of mid-infrared excesses in G and K giantsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2008Mark H. Jones ABSTRACT Using photometric data from the Two-Micron All-Sky Survey (2MASS) and GLIMPSE catalogues, I investigate the incidence of mid-infrared (mid-IR) excesses (,10 ,m) in G and K stars of luminosity class III. In order to obtain a large sample size, stars are selected using a near-IR colour,magnitude diagram. Sources which are candidates for showing mid-IR excess are carefully examined and modelled to determined whether they are likely to be G/K giants. It is found that mid-IR excesses are present at a level of (1.8 ± 0.4) × 10,3. While the origin of these excesses remains uncertain, it is plausible that they arise from debris discs around these stars. I note that the measured incidence is consistent with a scenario in which dust lifetimes in debris discs are determined by Poynting,Robertson drag rather than by collisions. [source] Is AGN feedback necessary to form red elliptical galaxies?MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008A. Khalatyan ABSTRACT We have used the smoothed particle hydrodynamics (SPH) code gadget-2 to simulate the formation of an elliptical galaxy in a group-size cosmological dark matter halo with mass Mhalo, 3 × 1012 h,1 M, at z= 0. The use of a stellar population synthesis model has allowed us to compute magnitudes, colours and surface brightness profiles. We have included a model to follow the growth of a central black hole and we have compared the results of simulations with and without feedback from active galactic nuclei (AGN). We have studied the interplay between cold gas accretion and merging in the development of galactic morphologies, the link between colour and morphology evolution, the effect of AGN feedback on the photometry of early-type galaxies, the redshift evolution in the properties of quasar hosts, and the impact of AGN winds on the chemical enrichment of the intergalactic medium (IGM). We have found that the early phases of galaxy formation are driven by the accretion of cold filamentary flows, which form a disc galaxy at the centre of the dark matter halo. Disc star formation rates in this mode of galaxy growth are about as high as the peak star formation rates attained at a later epoch in galaxy mergers. When the dark matter halo is sufficiently massive to support the propagation of a stable shock, the gas in the filaments is heated to the virial temperature, cold accretion is shut down, and the star formation rate begins to decline. Mergers transform the spiral galaxy into an elliptical one, but they also reactivate star formation by bringing gas into the galaxy. Without a mechanism that removes gas from the merger remnants, the galaxy ends up with blue colours, which are atypical for its elliptical morphology. We have demonstrated that AGN feedback can solve this problem even with a fairly low heating efficiency. Our simulations support a picture where AGN feedback is important for quenching star formation in the remnant of wet mergers and for moving them to the red sequence. This picture is consistent with recent observational results, which suggest that AGN hosts are galaxies in migration from the blue cloud to the red sequence on the colour,magnitude diagram. However, we have also seen a transition in the properties of AGN hosts from blue and star forming at z, 2 to mainly red and dead at z, 0. Ongoing merging is the primary but not the only triggering mechanism for luminous AGN activity. Quenching by AGN is only effective after the cold filaments have dried out, since otherwise the galaxy is constantly replenished with gas. AGN feedback also contributes to raising the entropy of the hot IGM by removing low-entropy tails vulnerable to developing cooling flows. We have also demonstrated that AGN winds are potentially important for the metal enrichment of the IGM a high redshift. [source] On the properties of young multiple starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2004E. J. Delgado-Donate ABSTRACT We present numerical results on the properties of young binary and multiple stellar systems. Our analysis is based on a series of smoothed particle hydrodynamics (SPH) +N -body simulations of the fragmentation of small molecular clouds, which fully resolve the opacity limit for fragmentation. These simulations demonstrate that multiple star formation is a major channel for star formation in turbulent flows. We have produced a statistically significant number of stable multiple systems, with component separations in the range ,1,103 au. At the end of the hydrodynamic stage (0.5 Myr), we find that ,60 per cent of stars and brown dwarfs are members of multiples systems, with about a third of these being low-mass, weakly bound outliers in wide eccentric orbits. Our results imply that in the stellar regime most stars are in multiples (,80 per cent) and that this fraction is an increasing function of primary mass. After N -body integration to 10.5 Myr, the percentage of bound objects has dropped to about 40 per cent, this decrease arising mostly from very low-mass stars and brown dwarfs that have been released into the field. Brown dwarfs are never found to be very close companions to stars (the brown dwarf desert at very small separations), but one case exists of a brown dwarf companion at intermediate separations (10 au). Our simulations can accommodate the existence of brown dwarf companions at large separations, but only if the primaries of these systems are themselves multiples. We have compared the outcome of our simulations with the properties of real stellar systems as deduced from the infrared colour,magnitude diagram of the Praesepe cluster and from spectroscopic and high-resolution imaging surveys of young clusters and the field. We find that the spread of the observed main sequence of Praesepe in the 0.4,1 M, range appears to require that stars are indeed commonly assembled into high-order multiple systems. Similarly, observational results from Taurus and , Ophiuchus, or moving groups such as TW Hydrae and MBM 12, suggest that companion frequencies in young systems can indeed be as high as we predict. The comparison with observational data also illustrates two problems with the simulation results. First, low mass ratio (q < 0.2) binaries are not produced by our models, in conflict with both the Praesepe colour,magnitude diagram and independent evidence from field binary surveys. Secondly, very low-mass stars and brown dwarf binaries appear to be considerably underproduced by our simulations. [source] Dwarf elliptical galaxies: structure, star formation and colour,magnitude diagramsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Giovanni Carraro The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N -body hydrodynamical simulations that include star formation, feedback and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the colour,magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment, thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large-scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present colour,magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the redshift of galaxy formation and varying the cosmological parameters H0 and q0. The resulting colour,magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group. [source] New brown dwarf candidates in the Pleiades,,ASTRONOMISCHE NACHRICHTEN, Issue 5 2009T. Eisenbeiss Abstract We have performed deep, wide-field imaging on a ,0.4 deg2 field in the Pleiades (Melotte 22). The selected field was not yet target of a deep search for low mass stars and brown dwarfs. Our limiting magnitudes are R , 22 mag and I , 20 mag, sufficient to detect brown dwarf candidates down to 40 MJ. We found 197 objects, whose location in the (I, R , I) color magnitude diagram is consistent with the age and the distance of the Pleiades. Using CTK R and I as well as JHK photometry from our data and the 2MASS survey we were able to identify 7 new brown dwarf candidates. We present our data reduction technique, which enables us to resample, calibrate, and co-add many images by just two steps. We estimate the interstellar extinction and the spectral type from our optical and the NIR data using a two-dimensional ,2 fitting (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] UBV(RI)C,JHK observations of Hipparcos -selected nearby starsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2010C. Koen ABSTRACT We present homogeneous, standardized UBV(RI)C photometry for over 700 nearby stars selected on the basis of Hipparcos parallaxes. Additionally, we list JHK photometry for about half of these stars, as well as L photometry for 86 of the brightest. A number of stars with peculiar colours or anomalous locations in various colour,magnitude diagrams are discussed. [source] An HST/ACS view of the inhomogeneous outer halo of M31,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2009J. C. Richardson ABSTRACT We present a high precision photometric view of the stellar populations in the outer halo of M31, using data taken with the Hubble Space Telescope/Advanced Camera for Surveys. We analyse the field populations adjacent to 11 luminous globular clusters which sample the galactocentric radial range 18 ,R, 100 kpc and reach a photometric depth of ,2.5 mag below the horizontal branch (mF814W, 27 mag). The colour,magnitude diagrams are well populated out to ,60 kpc and exhibit relatively metal-rich red giant branches, with the densest fields also showing evidence for prominent red clumps. We use the Dartmouth isochrones to construct metallicity distribution functions which confirm the presence of dominant populations with ,[Fe/H],,,0.6 to ,1.0 dex and considerable metallicity dispersions of 0.2 to 0.3 dex (assuming a 10 Gyr population and scaled-solar abundances). The average metallicity over the range 30,60 kpc is [Fe/H]=,0.80 ± 0.14 dex, with no evidence for a significant radial gradient. Metal-poor stars ([Fe/H],,1.3) typically account for ,10,20 per cent of the population in each field, irrespective of radius. Assuming our fields are unbiased probes of the dominant stellar populations in these parts, we find that the M31 outer halo remains considerably more metal rich than that of the Milky Way out to at least 60 kpc. [source] The stellar population content of the thick disc and halo of the Milky Way analogue NGC 891MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 3 2009M. Rejkuba ABSTRACT We present deep VI images obtained with the Advanced Camera for Surveys on board the Hubble Space Telescope, covering three fields in the north-east side of the edge-on disc galaxy NGC 891. The observed fields span a wide range of galactocentric distances along the eastern minor axis, extending from the plane of the disc to 12 kpc, and out to ,25 kpc along the major axis. The photometry of individual stars reaches ,2.5 mag below the tip of the red giant branch. We use the astrophotometric catalogue to probe the stellar content and metallicity distribution across the thick disc and spheroid of NGC 891. The colour,magnitude diagrams of thick disc and spheroid population are dominated by old red giant branch stars with a wide range of metallicities, from the sparsely populated metal-poor tail at [Fe/H],,2.4 dex, up to about half-solar metallicity. The peak of the metallicity distribution function of the thick disc is at ,0.9 dex. The inner parts of the thick disc, within ,14 kpc along the major axis show no vertical colour/metallicity gradient. In the outer parts, a mild vertical gradient of ,(V,I)0/,|Z| = 0.1 ± 0.05 kpc,1 or less than 0.1 dex kpc,1 is detected, with bluer colours or more metal-poor stars at larger distances from the plane. This gradient is, however, accounted for by the mixing with the metal-poor halo stars. No metallicity gradient along the major axis is present for thick-disc stars, but strong variations of about 0.35 dex around the mean of [Fe/H]=,1.13 dex are found. The properties of the asymmetric metallicity distribution functions of the thick-disc stars show no significant changes in both the radial and the vertical directions. The stellar populations situated within the solar-cylinder-like distances show strikingly different properties from those of the Galaxy populating similar distances. This suggests that the accretion histories of both galaxies have been different. The spheroid population, composed of the inner spheroid and the halo, shows remarkably uniform stellar population properties. The median metallicity of the halo stellar population shows a shallow gradient from about ,1.15 dex in the inner parts to ,1.27 dex at 24 kpc distance from the centre, corresponding to ,13reff. Similar to the thick-disc stars, large variations around the mean relation are present. [source] An analysis of the composite stellar population in M32,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009P. Coelho ABSTRACT We obtained long-slit spectra of high signal-to-noise ratio of the galaxy M32 with the Gemini Multi-Object Spectrograph at the Gemini-North telescope. We analysed the integrated spectra by means of full spectral fitting in order to extract the mixture of stellar populations that best represents its composite nature. Three different galactic radii were analysed, from the nuclear region out to 2 arcmin from the centre. This allows us to compare, for the first time, the results of integrated light spectroscopy with those of resolved colour,magnitude diagrams from the literature. As a main result we propose that an ancient and an intermediate-age population co-exist in M32, and that the balance between these two populations change between the nucleus and outside one effective radius (1reff) in the sense that the contribution from the intermediate population is larger at the nuclear region. We retrieve a smaller signal of a young population at all radii whose origin is unclear and may be a contamination from horizontal branch stars, such as the ones identified by Brown et al. in the nuclear region. We compare our metallicity distribution function for a region 1 to 2 arcmin from the centre to the one obtained with photometric data by Grillmair et al. Both distributions are broad, but our spectroscopically derived distribution has a significant component with [Z/Z,],,1, which is not found by Grillmair et al. [source] Seven young star clusters in the inner region of the Small Magellanic CloudMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2008Andrés E. Piatti ABSTRACT We present CCD photometry in the Washington system C and T1 passbands down to T1, 22 in the fields of L35, L45, L49, L50, L62, L63 and L85, seven poorly studied star clusters in the inner region of the Small Magellanic Cloud (SMC). We measured T1 magnitudes and C,T1 colours for a total of 114 826 stars distributed throughout cluster areas of 13.7 × 13.7 arcmin2 each. Cluster radii were estimated from star counts distributed throughout the entire observed fields. The seven clusters are generally characterized by a relatively small angular size and by a high field star contamination. We performed an in-depth analysis of the field star contamination of the colour,magnitude diagrams (CMDs), and statistically cleaned the cluster CMDs. Based on the best fits of isochrones computed by the Padova group to the (T1, C,T1) CMDs, we derive ages for the sample, assuming Z= 0.004, finding ages between 25 Myr and 1.2 Gyr. We then examined different relationships between positions in the SMC, age and metallicity of a larger sample of clusters including our previous work whose ages and metallicities are on the same scale used in this paper. We confirm previous results in the sense that the further a cluster is from the centre of the galaxy, the older and more metal poor it is, with some dispersion; although clusters associated with the Magellanic Bridge clearly do not obey the general trend. The number of clusters within , 2° of the SMC centre appears to have increased substantially after ,2.5 Gyr ago, hinting at a burst. [source] The stellar content of the isolated transition dwarf galaxy DDO210,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2006Alan W. McConnachie ABSTRACT We use Subaru Suprime-Cam and VLT FORS1 photometry of the dwarf galaxy DDO210 to study the global stellar content and structural properties of a transition-type galaxy (with properties intermediate between dwarf irregular and dwarf spheroidal systems). This galaxy is sufficiently isolated that tidal interactions are not likely to have affected its evolution in any way. The colour,magnitude diagrams of DDO210 show a red giant branch (RGB) population (with an RGB bump), a bright asymptotic giant branch population, a red clump, young main-sequence stars and blue-loop stars. The youngest stars formed within the last 60 Myr and have a distinct radial distribution compared to the main population. Whereas the overall stellar spatial distribution and H i spatial distribution are concentric, the young stars are offset from the centre of DDO210 and are coincident with a ,dent' in the H i distribution. The implied recent star formation rate required to form the young population is significantly higher than the derived current star formation rate, by a factor of >10. Most of the stars in DDO210 are found in a red clump, and its mean I -band magnitude suggests that the majority of stars in DDO210 have an average age of 4+2,1 Gyr. Given this age, the colour of the RGB implies a mean metallicity of [Fe/H],,1.3. By comparing the shape of the red clump with models for a variety of star formation histories, we estimate that an old (>10 Gyr) stellar population can contribute ,20,30 per cent of the stars in DDO210 at most. The unusual star formation history of DDO210, its low-mass estimate and its isolated nature, provide insight into how star formation proceeds in the lowest mass, unperturbed, dwarf galaxy haloes. [source] Discovery and analysis of three faint dwarf galaxies and a globular cluster in the outer halo of the Andromeda galaxy,MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2006N. F. Martin ABSTRACT We present the discovery of three faint dwarf galaxies and a globular cluster in the halo of the Andromeda galaxy (M31), found in our MegaCam survey that spans the southern quadrant of M31, from a projected distance of ,50 to ,150 kpc. Though the survey covers 57 deg2, the four satellites lie within 2° of one another. From the tip of the red giant branch (RGB), we estimate that the globular cluster lies at a distance of 631 ± 58 kpc from the Milky Way and along with a ,100 kpc projected distance from M31 we derive a total distance of 175 ± 55 kpc from its host, making it the farthest M31 globular cluster known. It also shows the typical characteristics of a bright globular cluster, with a half-light radius of 2.3 ± 0.2 pc and an absolute magnitude in the V band of MV,0=,8.5 ± 0.3. Isochrone fitting reveals that it is dominated by a very old population with a metallicity of [Fe/H],,1.3. The three dwarf galaxies are revealed as overdensities of stars that are aligned along the RGB tracks in their colour,magnitude diagrams. These satellites are all very faint, with absolute magnitudes in the range ,7.3 ,MV,0,,6.4, and show strikingly similar characteristics with metallicities of [Fe/H],,1.4 and half-light radii of ,120 ± 45 pc, making these dwarf galaxies two to three times smaller than the smallest previously known satellites of M31. Given their faintness, their distance is difficult to constrain, but we estimate them to be between 740 and 955 kpc which places them well within the virial radius of the host galaxy. The panoramic view of the MegaCam survey can provide an unbiased view of the satellite distribution of the Andromeda galaxy and, extrapolating from its coverage of the halo, we estimate that up to 45 ± 20 satellites brighter than MV,,6.5 should be orbiting M31. Hence faint dwarf galaxies cannot alone account for the missing satellites that are predicted by , cold dark matter models, unless they reside in dark matter minihaloes that are more massive than the typical masses of 107 M, currently inferred from their central radial velocity dispersion. [source] The lithium depletion boundary and the age of NGC 2547MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2003J. M. Oliveira ABSTRACT We present the results of a photometric and spectroscopic survey of cool M dwarf candidates in the young open cluster NGC 2547. Using the 2dF fibre spectrograph, we have searched for the luminosity at which lithium remains unburned in an attempt to constrain the cluster age. The lack of a population of individual lithium-rich objects towards the faint end of our sample places a very strong lower limit to the cluster age of 35 Myr. However, the detection of lithium in the averaged spectra of our faintest targets suggests that the lithium depletion boundary lies at 9.5 < MI < 10.0 and that the cluster age is <54 Myr. The age of NGC 2547 judged from fitting isochrones to low-mass pre-main-sequence stars in colour,magnitude diagrams is 20,35 Myr using the same evolutionary models. The sense and size of the discrepancy in age determined by these two techniques is similar to that found in another young cluster, IC 2391, and in the low-mass pre-main-sequence binary system, GJ 871.1AB. We suggest that the inclusion of rotation or dynamo-generated magnetic fields in the evolutionary models could reconcile the two age determinations, but only at the expense of increasing the cluster ages beyond that currently indicated by the lithium depletion. Alternatively, some mechanism is required that increases the rate of lithium depletion in young, very low-mass fully convective stars. [source] Dwarf elliptical galaxies: structure, star formation and colour,magnitude diagramsMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 1 2001Giovanni Carraro The aim of this paper is to cast light on the formation and evolution of elliptical galaxies by means of N -body hydrodynamical simulations that include star formation, feedback and chemical evolution. Particular attention is paid to the case of dwarf spheroidals of the Local Group which, thanks to their proximity and modern ground-based and space instrumentation, can be resolved into single stars so that independent determinations of their age and star formation history can be derived. Indeed, the analysis of the colour,magnitude diagram of their stellar content allows us to infer the past history of star formation and chemical enrichment, thus setting important constraints on galactic models. Dwarf galaxies are known to exhibit complicated histories of star formation ranging from a single very old episode to a series of bursts over most of the Hubble time. By understanding the physical process driving star formation in these objects, we might be able to infer the mechanism governing star formation in more massive elliptical galaxies. Given these premises, we start from virialized haloes of dark matter, and follow the infall of gas into the potential wells and the formation of stars. We find that in objects of the same total mass, different star formation histories are possible, if the collapse phase started at different initial densities. We predict the final structure of dwarf spheroidal galaxies, their kinematics, their large-scale distribution of gas and stars, and their detailed histories of the star formation and metal enrichment. Using a population synthesis technique, star formation and metal enrichment rates are then adopted to generate the present colour,magnitude diagrams of the stellar populations hosted by dwarf spheroidal galaxies. The simulations are made assuming the redshift of galaxy formation and varying the cosmological parameters H0 and q0. The resulting colour,magnitude diagrams are then compared with the observational ones for some dwarf spheroidals of the Local Group. [source] The effect of stellar rotation on colour,magnitude diagrams: on the apparent presence of multiple populations in intermediate age stellar clustersMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2009N. Bastian ABSTRACT A significant number of intermediate age clusters (1,2 Gyr) in the Magellanic Clouds appear to have multiple stellar populations within them, derived from bimodal or extended main-sequence turn-offs. If this is interpreted as an age spread, the multiple populations are separated by a few hundred million years, which would call into question the long-held notion that clusters are simple stellar populations. Here, we show that stellar rotation in stars with masses between 1.2 and 1.7 M, can mimic the effect of a double or multiple population, whereas in actuality only a single population exists. The two main causes of the spread near the turn-off are the effects of stellar rotation on the structure of the star and the inclination angle of the star relative to the observer. Both effects change the observed effective temperature, hence colour, and flux of the star. In order to match observations, the required rotation rates are 20,50 per cent of the critical rotation, which are consistent with observed rotation rates of similar mass stars in the Galaxy. We provide scaling relations which can be applied to non-rotating isochrones in order to mimic the effects of rotation. Finally, we note that rotation is unlikely to be the cause of the multiple stellar populations observed in old globular clusters, as low-mass stars (<1 M,) are not expected to be rapid rotators. [source] | ||||||||||