Model Color-magnitude Diagrams Research Articles

POPULATION EFFECTS ON THE METALLICITY DISTRIBUTION FUNCTION DERIVED FROM THE RED GIANT BRANCH

We have tested the reliability of the red giant branch (RGB) as a metallicity indicator accounting for observational errors as well as the complexity of star formation histories (SFHs) and chemical evolution histories observed in various stellar systems. We generate model color-magnitude diagrams (CMDs) produced with a variety of evolutionary histories and compare the resultant metallicity estimates from the colors and magnitudes of RGB stars to the true input metallicities. We include realistic models for photometric errors and completeness in our synthetic CMDs. As expected, for simple simple stellar populations dominated by old stars, the RGB provides a very accurate estimate of the modular metallicity value for a population. An error in the age of a system targeted for this type of study may produce metallicity errors of a few tenths of a dex. The size of this metallicity error depends linearly on the age error, and we find this dependence to be stronger with more precise photometry. If the population has experienced any significant star formation within the last $\sim$6 Gyr, the metallicity estimates, [M/H], derived from the RGB may be in error by up to $\sim$0.5 dex. Perhaps the most important consideration for this technique is an accurate, independent estimate of the average age for the target stellar system, especially if it is probable that a significant fraction of the population formed less than $\sim$6 Gyr ago.

Testing substellar models with dynamical mass measurements

We have been using Keck laser guide star adaptive optics to monitor the orbits of ultracool binaries, providing dynamical masses at lower luminosities and temperatures than previously available and enabling strong tests of theoretical models. We have identified three specific problems with theory: (1) We find that model color–magnitude diagrams cannot be reliably used to infer masses as they do not accurately reproduce the colors of ultracool dwarfs of known mass. (2) Effective temperatures inferred from evolutionary model radii are typically inconsistent with temperatures derived from fitting atmospheric models to observed spectra by 100–300 K. (3) For the only known pair of field brown dwarfs with a precise mass (3%) and age determination (≈25%), the measured luminosities are ~2–3× higher than predicted by model cooling rates (i.e., masses inferred from L bol and age are 20–30% larger than measured). To make progress in understanding the observed discrepancies, more mass measurements spanning a wide range of luminosity, temperature, and age are needed, along with more accurate age determinations (e.g., via asteroseismology) for primary stars with brown dwarf binary companions. Also, resolved optical and infrared spectroscopy are needed to measure lithium depletion and to characterize the atmospheres of binary components in order to better assess model deficiencies.

FAKE STAR FORMATION BURSTS: BLUE HORIZONTAL BRANCH STARS MASQUERADE AS YOUNG MASSIVE STARS IN OPTICAL INTEGRATED LIGHT SPECTROSCOPY

Model color magnitude diagrams of low-metallicity globular clusters usually show a deficit of hot evolved stars with respect to observations. We investigate quantitatively the impact of such modelling inaccuracies on the significance of star formation history reconstructions obtained from optical integrated spectra. To do so, we analyse the sample of spectra of galactic globular clusters of Schiavon et al. with STECKMAP (Ocvirk et al.) and the stellar population models Vazdekis et al. and Bruzual & Charlot, and focus on the reconstructed stellar age distributions. Firstly, we show that background/foreground contamination correlates with E(B-V), which allows us to define a clean subsample of uncontaminated GCs, on the basis of a E(B-V) filtering. We then identify a "confusion zone" where fake young bursts of star formation pop up in the star formation history although the observed population is genuinely old. These artifacts appear for 70-100% of cases depending on the population model used, and contribute up to 12% of the light in the optical. Their correlation with the horizontal branch ratio indicates that the confusion is driven by HB morphology: red horizontal branch clusters are well fitted by old stellar population models while those with a blue HB require an additional hot component. The confusion zone extends over [Fe/H]=[-2,-1.2], although we lack the data to probe extreme high and low metallicity regimes. As a consequence, any young starburst superimposed on an old stellar population in this metallicity range could be regarded as a modeling artifact, if it weighs less than 12% of the optical light, and if no emission lines typical of an HII region are present.

Testing Models with Brown Dwarf Binaries

AbstractWe have been using Keck laser guide star adaptive optics to monitor the orbits of ultracool binaries, providing dynamical masses at lower luminosities and temperatures than previously available and enabling strong tests of theoretical models. (1) We find that model color–magnitude diagrams cannot reliably be used to infer masses as they do not accurately reproduce the colors of ultracool dwarfs of known mass. (2) Effective temperatures inferred from evolutionary model radii can be inconsistent with temperatures derived from fitting observed spectra with atmospheric models by at most 100–300 K. (3) For the single pair of field brown dwarfs with a precise mass (3%) and age determination (≈25%), the measured luminosities are ~2–3× higher than predicted by model cooling rates (masses inferred from Lbol and age are 20–30% larger than measured). Finally, as the sample of binaries with measured orbits grows, novel tests of brown dwarf formation theories are made possible (e.g., testing theoretical eccentricity distributions).

The ACS LCID project: accurate measurements of the full star formation history in low metallicity, isolated, Local Group dwarf galaxies

AbstractWe present here the latest results of the LCID project (Local Cosmology from Isolated Dwarfs), aimed at recovering the full star formation history (SFH) of six isolated dwarf galaxies of the Local Group (LG). Our method of analysis is based on the IAC-pop code, which derives the SFH of a resolved stellar system by comparing the observed and a model color-magnitude diagram (CMD). We summarize here basic technical issues and the main results concerning our sample of galaxies. We show that LeoA is the only object showing a clear delay in the onset of the major SF event, while all the other galaxies present a dominant component older than 10 Gyrs.

Models for the Observable System Parameters of Ultraluminous X‐Ray Sources

We investigate the evolution of model populations of ultraluminous X-ray sources (ULXs), consisting of a black hole accretor in a binary with a donor star. Two of the models we consider invoke stellar-mass (up to ~25 M☉) black hole binaries (LMBHs), generated with a binary population synthesis code, while a third model uses intermediate-mass (~1000 M☉) black hole accretors (IMBHs). For each model, we computed 30,000 binary evolution sequences. A scheme for calculating the optical flux from ULXs is discussed. We present ``probability images'' for the color-magnitude diagrams (CMDs) and for the orbital period-X-ray luminosity (Porb-Lx) plane. We show how a population of luminous X-ray sources in a cluster of stars evolves with time. The most probable ULX system parameters correspond to high-mass donors (of initial mass ≳25 M☉) with effective O through late B spectral types, and Porb between 1 and 10 days. Estimates of the numbers of ULXs in a typical galaxy as a function of Lx are also presented. We find that if LMBHs are allowed to have super-Eddington Lx, the binding energy parameter for the stellar envelope of the black hole progenitor must be λ ≲ 0.03 in order not to overproduce ULXs. Comparison of six known ULX counterparts with our model CMDs indicates that the IMBH model somewhat more closely matches the observations. We find that a significant contribution to the optical flux from the IMBH systems comes from intrinsic accretion disk radiation. In effect, IMBH systems, when operating at their maximum luminosities (1041-1042 ergs s−1), are milli-AGNs. While models of IMBH systems during the X-ray phase are attractive, their formation mechanism remains uncertain.

Mass segregation in rich LMC clusters from modelling of deep HST colour–magnitude diagrams

Aims. We used the deep colour–magnitude diagrams (CMDs) of five rich LMC clusters (NGC 1805, NGC 1818, NGC 1831, NGC 1868, and Hodge 14) observed with HST/WFPC2 to derive their present day mass function (PDMF) and its variation with position within the cluster. Methods. The PDMF was parameterized as a power law in the available main-sequence mass range of each cluster, typically 0.9 < m/M� < 2.5; its slope was determined at different positions spanning from the very centre out to several core radii. The CMDs in the central regions of the clusters were carefully studied earlier, resulting in accurate age, metallicity, distance modulus, and reddening values. The slope α (where Salpeter is 2.35) was determined in annuli by following two distinct methods: 1) a power law fit to the PDMF obtained from the systemic luminosity function (LF); 2) a statistical comparison between observed and model CMDs. In the second case, α is a free input parameter in the CMD modelling process where we incorporate photometric errors and the effect of binarity as a fraction of unresolved binaries (fbin = 100%) with random pairing of masses from the same PDMF. Results. In all clusters, significant mass segregation is found from the positional dependence of the PDMF slope: α < 1. 8f or R ≤ 1.0 Rcore and α ∼ Salpeter inside R = 2 ∼ 3 Rcore (except for Hodge 14, where α ∼ Salpeter for R ∼ 4 Rcore). The results are robust in the sense that they hold true for both methods used. The CMD method reveals that unresolved binaries flatten the PDMF obtained form the systemic LF, but this effect is smaller than the uncertainties in the α determination. For each cluster we estimated dynamical ages inside the core and for the entire system. In both cases we found a trend in the sense that older clusters have flatter PDMF, consistent with a dynamical mass segregation and stellar evaporation.

The Star Formation History of the Local Group Dwarf Galaxy Leo I

We present a quantitative analysis of the star formation history (SFH) of the Local Group dSph galaxy Leo I, from the information in its Hubble Space Telescope [(V-I), I] color-magnitude diagram (CMD). It reaches the level of the oldest main-sequence turnoffs, and this allows us to retrieve the SFH in considerable detail. The method we use is based on comparing, via synthetic CMDs, the expected distribution of stars in the CMD for different evolutionary scenarios with the observed distribution. We consider the SFH to be composed by the SFR(t), the chemical enrichment law Z(t), the initial mass function (IMF), and a function β(f, q) controlling the fraction f and mass ratio distribution q of binary stars. We analyze a set of ≃50 combinations of four Z(t), three IMFs, and more than four β(f, q). For each of them, the best SFR(t) is searched for among ≃6 × 107 models. The comparison between the observed CMD and the model CMDs is done through χ minimization of the differences in the number of stars in a set of regions of the CMD, chosen to sample stars of different ages or in specific stellar evolutionary phases. We empirically determine the range of χ values that indicate acceptable models for our set of data using tests with models with known SFHs. Our solution for the SFH of Leo I defines a minimum of χ in a well-defined position of the parameter space, and the derived SFR(t) is robust, in the sense that its main characteristics are unchanged for different combinations of the remaining parameters. However, only a narrow range of assumptions for Z(t), IMF, and β(f, q) result in a good agreement between the data and the models, namely, Z = 0.0004, a IMF Kroupa et al. or slightly steeper, and a relatively large fraction of binary stars, with f = 0.3–0.6, q > 0.6, and an approximately flat IMF for the secondaries, or particular combinations of these parameters that would produce a like fraction of similar mass binaries. Most star formation activity (70% to 80%) occurred between 7 and 1 Gyr ago. At 1 Gyr ago, it abruptly dropped to a negligible value, but seems to have been active until at least ≃300 million years ago. Our results do not unambiguously answer the question of whether Leo I began forming stars around 15 Gyr ago, but it appears that the amount of this star formation, if it existed at all, would be small.

The Star Formation History of the Magellanic Clouds: A Preliminary Report

We present a method to determine the star formation history (SFH) of a mixed stellar population, based on an iterative maximum Likelihood comparison of stellar photometry to model color-magnitude diagrams. We demonstrate the algorithm on a subregion of the Large Magellanic Cloud, observed as part of our ongoing Magellanic Clouds Photometric Survey. We will eventually perform this analysis on hundreds of regions in both Clouds, resulting in a homogeneous SFH map of these galaxies.

The Stellar Content and the Star Formation History of the Local Group Dwarf Galaxy LGS 3.

view Abstract Citations (77) References (35) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Stellar Content and the Star Formation History of the Local Group Dwarf Galaxy LGS 3. Aparicio, A. ; Gallart, C. ; Bertelli, G. Abstract The star formation history (SFH) and the properties of the dwarf galaxy LGS 3 are analyzed using color-magnitude (CM) diagrams plotted from VRI photometry of 736 stars. The distance to the galaxy is estimated through the position of the tip or the red giant branch. Two acceptable results have been obtained: 0.77+/- 0.07 Mpc and 0.96+/- 0.07 Mpc, although the first value is favored by complementary considerations on the stellar content of the galaxy. Both values make LGS 3 a possible satellite of M31 or of M33. The SFH is investigated for each of the two adopted distances comparing the observed (CM) diagrams with model CM diagrams computed for different star formation rates (psi (t)) and chemical enrichment laws (Z(t)). The results are compatible with LGS 3 having been forming stars since an early epoch, 15-12 Gyr ago, at an almost constant rate if distance is 0.77 Mpc or at an exponentially decreasing rate if distance is 0.96 Mpc. According to our models, the current metallicity would range from Z =~ 0.0007 to Z =~ 0.002. Other results are the current psi (t): (0.55+/- 0.04)x 10(-10) M_sun yr(-1) pc(-2) or (0.47+/- 0.07)x 10(-10) M_sun yr(-1) pc(-2) , depending of the distance, and its average for the entire life of the galaxy, bar psi =(1.4+/- 0.1)x 10(-10) M_sun yr(-1) pc(-2) . At the present psi (t), the probability of LGS 3 having an HII region is 0.2, which is compatible with the fact that no HII regions have been found in the galaxy. Its fraction of gas relative to the mass intervening in the chemical evolution is about 0.40 and its percentage of dark matter (that which cannot be explained as stellar remnants or by extrapolation of the used IMF to low masses) is 95%. The results for psi (t) and Z(t) for d=0.77 Mpc are compatible with a moderate outflow of well mixed material (lambda =3), but large outflow rate (lambda =30) is required to account for the results for d=0.96 Mpc. The latter would imply that, if the amount of dark mass associated to the galaxy is constant, the initial dark matter fraction was about 50%. In both cases, a large fraction of freshly made metals (gamma =0.85 for the case of d=0.77 Mpc and gamma =0.36 for d=0.96 Mpc) should also escape from the galaxy before mixing with the interstellar medium to make the results compatible with the theoretical yields. LGS 3 is considered an intermediate type between dIr and dE. However we find that it shows characteristics typical of dIrs (the M_gas/L_B ratio and the SFH shape), the main difference being that its mass and star formation rate (present and averaged) are one to two orders of magnitude smaller than those of other dIrs. This makes the absence of HII regions a simple probabilistic effect. Considering this and the fact that LGS 3 can continue to form stars for a further 10 Gyr at a rate equal to that averaged for its past history, we conclude that this galaxy may be considered just a dIr in the tail of the distributions of mass, luminosity and star formation rate. Publication: The Astronomical Journal Pub Date: August 1997 DOI: 10.1086/118502 arXiv: arXiv:astro-ph/9705005 Bibcode: 1997AJ....114..680A Keywords: GALAXIES: PECULIAR; GALAXIES: STELLAR CONTENT; STARS: FORMATION; Astrophysics E-Print: 23 pages, 3 tables, 14 figures. AJ in press full text sources arXiv | ADS | data products SIMBAD (6) NED (1)

On the Origin of the UV Upturn in Elliptical Galaxies. I. Sensitivity of UV Population Synthesis to Various Input Parameters

We present models of the late stages of stellar evolution intended to explain the UV upturn phenomenon in elliptical galaxies. Such models are sensitive to values of a number of poorly-constrained physical parameters, including metallicity, age, stellar mass loss, helium enrichment, and the distribution of stars on the zero age horizontal branch (HB). We explore the sensitivity of the results to values of these parameters, and reach the following conclusions. Old, metal rich galaxies, such as giant ellipticals, naturally develop a UV upturn within a reasonable time scale - less than a Hubble time - without the presence of young stars. The most likely stars to dominate the UV flux of such populations are low mass, core helium burning (HB and evolved HB) stars. Metal-poor populations produce a higher ratio of UV-to-V flux, due to opacity effects, but only metal-rich stars develop a UV upturn, in which the flux increases towards shorter UV wavelengths. Model color-magnitude diagrams and corresponding integrated spectra (for various values of age, metallicity, helium enrichment, mass loss efficiency, initial mass function, and the HB mass dispersion factor) are available on S.Y.'s world wide web site http://shemesh.gsfc.nasa.gov/model.html.

Luminous Red Stars in Local Group Dwarf Elliptical Galaxies: An Intermediate-Age Population?

In this paper, we show that the optically bright stars above the tip of the first red giant branch (TRGB) in the color magnitude (CM) diagrams of nearby dwarf elliptical (dE) galaxies, commonly interpreted as indication of the existence of an intermediate-age population, may, in certain circumstances, be an artifact of observational effects on a pure old population system. For this purpose, model CM diagrams have been computed simulating the observational effects that can be found in different regions of a Local Group dE galaxy. The starting synthetic CM diagram represents a pure old system, with star formation extending only from 15 to 12 Gyr ago. Based on those model diagrams, we analyze the conclusions that a hypothetical ground-based observer might reach concerning the presence of a population of stars above the tip of the red giant branch (TRGB) that could be interpreted as an intermediate-age population, by using two age indicators extensively employed in the literature: i) the bolometric magnitude at the tip of the asymptotic giant branch (AGB) phase, and ii) the ratio of AGB to red giant branch (RGB) stars present in the CM diagram. Our analysis shows that, if observational effects are overlooked, application of both methods would result in finding a fictitious intermediate-age population in the inner regions of the galaxy, where crowding is more severe.

OLD POPULATIONS AND THE STAR FORMATION HISTORY OF THE LOCAL GROUP DWARF IRREGULAR GALAXY NGC 6822

ABSTRACT A method for studying the full star formation history (SFH) of nearby galaxies,as a function of time, has been developped and implemented. For the first time, the full SFH of a nearby dwarf irregular galaxy (DIG) has been retrieved from the photometry of its resolved stars. NGC~6822 has been observed in UBVRI at the 2.5m INT telescope, at the Roque de los Muchachos Observatory and photometry of the resolved stellar content has been obtained [1]. The VRI frames are very deep and contain about 18000 well measured stars. Most of them are old stars populating the red-giant branch (RGB) and the asymptotic giant branch (AGB) phase, which has been well sampled. Using the new BVRI magnitudes for the Cepheids, the first self-consistent BVRI multi-wavelength Cepheid distance and reddening has been derived for NGC~6822 [1]. The values obtained are (m-M)0=23.49± 0.08 and E(B-V)=0.24±0.03, respectively. The comparison of the [(V-I),I] and [(V-R),V] colour-magnitude (CM) diagrams with theoretical isochrones allowed the qualitative understanding of the stellar populations present in NGC~6822 ([1] and [4]). Two structures, the red-tangle and the red-tail ({tocho} and {colita}, as we originally called them) have been identified as the locus of the old and intermediate-age stars (age >1 Gyr), and therefore, as the structures to be studied to retrieve information of the old and intermediate-age SFH. Since stars older and younger than 1 Gyr are situated in clearly distinct areas of the CM diagram, and because of the different time-resolution on which these populations can be studied, the old and intermediate-age SFH and the young SFH have been derived separately ([2] and [3]). Nevertheless, the tool has been in both cases the same: the comparison of the observed [(V-I),I] CM diagram with a set of model CM diagrams, computed assuming different evolutionary scenarios. This is the main core of this thesis. The comparison has been performed through a number of indicators relative to the stars' distribution over the CM diagram. The model CM diagrams have been computed in collaboration with the Padova Group of stellar evolution. The reliability of the model CM diagrams computed is a result of three main components : i) a reliable set of stellar evolutionary models covering the needed range of ages and metallicities, ii) the interpolation of the stellar evolutionary tracks of fixed mass and metallicity to determine the precise, {smooth} distribution of stars of any age, mass, and metallicity on the CM diagram, and iii) a realistic simulation of the observational effects on the synthetic CM diagram. Significant constraints have been put on the SFH of NGC~6822. The main conclusions reached are the following: --Star formation beginning at Ti≤ 6 Gyr can be ruled out. Star formation beginning at Ti≃ 6 Gyr would only be possible if the galaxy started its early star formation from gas already enriched, or if a prompt metal enrichment occurred in the galaxy. --NGC~6822 has most likely begun forming stars at a very early epoch (about 15-12 Gyr ago), from low metallicity gas. --An SFR close to constant or declining in the last few Gyr of the galaxy's lifetime seems best to reproduce the observations. Short-time and small-amplitude fluctuations of the SFR may have occurred. -- An overall enhancement of the star formation activity has occurred in the last 100-200 Myr. The strength of this enhancement has been somewhat different from one region of the galaxy to another. Although applied to the case of a DIG, the fundamentals of the method developed in this thesis can be applied to galaxies of any morphological type, provided that deep enough photometry is available. This can be achieved, from the ground, for all Local Group galaxies, and using Hubble Space Telescope, for galaxies within 4--5 Mp.

THEORETICAL COLOR CALIBRATION FOR HST FOC ULTRAVIOLET FILTERS

ABSTRACT A theoretical color calibration is carried out for three (F140W, F220W, and F342W) of the most heavily used ultraviolet filters, and one visual band filter (F480LP) in the Faint Object Camera on the HST, using the new Kurucz model spectra which became available recently. It is known that the UV filters suffer from the visible leak problem, and it is apparent in this study that the visible leak problem can cause serious trouble to analyses of observed data. We show here that some of the colors defined from these filters do not have a monotonic relation with effective temperature; and thus, the colors fail to indicate a unique effective temperature in certain temperature ranges. This study shows that the problem can be solved by using one of the visual band filters, preferably F480LP, together with the UV filters. The expected model color-magnitude diagrams for a typical globular cluster with the HST ultraviolet colors are also provided.