Observed Colour Magnitude Diagrams Research Articles

Models of metal-poor stars with different initial abundances of C, N, O, Mg, and Si – I. Bolometric corrections derived from new MARCS synthetic spectra and their implications for observed colour–magnitude diagrams

ABSTRACTNew, high-resolution MARCS synthetic spectra have been calculated for more than a dozen mixtures of the metals allowing, in turn, for variations in C:N:O, [CNO/Fe], and enhanced abundances of C, O, Mg, and Si. Bolometric corrections (BCs) for many of the broad-band filters currently in use have been generated from these spectra. Due to improved treatments of molecules that involve atoms of C, N, and O, the BCs for UV and blue passbands, in particular, differ substantially from those derived from previous MARCS models. These differences, and the effects on the BCs of varying the abundances of the metals, are shown in a number of instructive plots. Stellar evolutionary grids for −2.5 ≤ [Fe/H] ≤−0.5 have also been computed for the different mixtures. Isochrones based on these tracks are intercompared on the theoretical H–R diagram and on a few of the colour–magnitude diagrams that can be constructed from HST Wide Field Camera 3 (WFC3) F336W, F438W, F606W, F814W, F110W, and F160W observations. For the first application of these models, isochrones have been fitted to WFC3 photometry of the globular cluster NGC 6496 from the HST UV Legacy Survey, with very encouraging results.

Models for metal-poor stars with different initial abundances of C, N, O, Mg, and Si. II. Application to the colour–magnitude diagrams of the globular clusters 47 Tuc, NGC 6362, M 5, M 3, M 55, and M92

ABSTRACTStellar models for −2.5 ≤ [Fe/H] ≤−0.5 that have been computed for variations in the C:N:O abundance ratio (for two different values of [CNO/Fe]) are compared with HST Wide Field Camera 3 (WFC3) observations of the globular clusters (GCs) 47 Tuc, NGC 6362, M 5 M 3, M 55, and M 92. The bolometric corrections (BCs) used to transpose the models to the observed planes are based on new MARCS synthetic spectra that incorporate improved treatments of molecules that involve atoms of C, N, and O. On the assumption of well-supported distance moduli and reddenings, isochrones for [O/Fe] = 0.6 and [m/Fe] = 0.4 for the other α elements, which are favoured by binary stars in GCs, generally reproduce the main features of observed colour–magnitude diagrams (CMDs) to within ∼0.03 mag. In particular, they appear to match the spreads in the observed (MF336W − MF438W)0 colours that are spanned by CN-weak and CN-strong stars along the lower giant branch quite well, but not the bluest giants, which are suspected to be N-poor ([N/Fe] ≲ −0.5). Both the absolute (MF438W − MF606W)0 colours and the variations in these colours at a given MF606W magnitude on the giant branch are difficult to explain unless the reddest stars are C-rich ([C/Fe] ≳ +0.5). Allowing for moderate He abundance variations ($\delta \, Y \sim 0.05$) improves the fits to the observations.

Is Fornax 4 the nuclear star cluster of the Fornax dwarf spheroidal galaxy?

ABSTRACT Fornax 4 is the most distinctive globular cluster in the Fornax dwarf spheroidal. Located close to the centre of the galaxy, more metal-rich and potentially younger than its four companions (namely, Fornax clusters number 1, 2, 3, and 5), it has been suggested to have experienced a different formation than the other clusters in the galaxy. Here, we use Hubble Space Telescope/WFC3 photometry to characterize the stellar population content of this system and shed new light on its nature. By means of a detailed comparison of synthetic horizontal branch and red giant branch with the observed colour–magnitude diagrams, we find that this system likely hosts stellar sub-populations characterized by a significant iron spread up to Δ[Fe/H] ∼ 0.4 dex and possibly by also some degree of He abundance variations ΔY ∼ 0.03. We argue that this purely observational evidence, combined with the other peculiarities characterizing this system, supports the possibility that Fornax 4 is the nuclear star cluster of the Fornax dwarf spheroidal galaxy. A spectroscopic follow-up for a large number of resolved member stars is needed to confirm this interesting result and to study in detail the formation and early evolution of this system and more in general the process of galaxy nucleation.

Photometric study of the young open clusters IC 1442, King 21, and Trumpler 7

ABSTRACT We carried out the UBVRcIc photometric study of three poorly studied young open clusters IC 1442, King 21, and Trumpler 7 (Tr 7). We obtained 263, 244, and 128 member stars using Gaia Data Release 2 proper motions and parallaxes in IC 1442, King 21, and Tr 7, respectively. The reddening, E(B − V), was derived to be 0.54 ± 0.04, 0.76 ± 0.06, and 0.38 ± 0.04 mag for these clusters. The comparison of observed colour–magnitude diagrams with solar metallicity isochrones yields log(age) = 7.40 ± 0.30, 7.70 ± 0.20, and 7.85 ± 0.25 yr and corresponding distances of 2847 ± 238, 2622 ± 156, and 1561 ± 74 pc for IC 1442, King 21, and Tr 7, respectively. The estimated mass function (MF) slopes are found to be −1.94 ± 0.18, −1.54 ± 0.32, and −2.31 ± 0.29 for IC 1442, King 21, and Tr 7, respectively. The study of MF slopes done separately in the inner and the outer regions of these clusters gives a steeper slope in the outer region, which suggests spatial variation in slope and mass segregation in the clusters. We found evidence of mass segregation after the dynamical study in these clusters. The obtained relaxation time, TE, is 74, 26, and 34 Myr for the clusters IC 1442, King 21, and Tr 7, respectively. The mass segregation in IC 1442 may be caused by early dynamical relaxation. The estimated TE is well below the ages of King 21 and Tr 7, which indicates that these clusters are dynamically relaxed.

A comprehensive study of open clusters Czernik 14, Haffner 14, Haffner 17 and King 10 using multicolour photometry and Gaia DR2 astrometry

ABSTRACT This paper presents an investigation into the four open clusters Czernik 14, Haffner 14, Haffner 17 and King 10, located near the Perseus arm of the Milky Way, using the Gaia DR2, 2MASS, WISE, APASS and Pan-STARRS1 data sets. We find normal interstellar extinction in 12 photometric bands for these clusters. Using Gaia DR2 proper motion data, the numbers of likely cluster members are found to be 225, 353, 350 and 395 for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Radii are determined as 3.5, 3.7, 6.2 and 5.7 arcmin for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Mean proper motions in RA and Dec. are estimated as (−0.42 ± 0.02, −0.38 ± 0.01), (−1.82 ± 0.009, 1.73 ± 0.008), (−1.17 ± 0.007, 1.88 ± 0.006) and (−2.75 ± 0.008, −2.04 ± 0.006) mas yr−1 for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. The comparison of observed colour–magnitude diagrams with solar metallicity isochrones leads to ages of 570 ± 60, 320 ± 35, 90 ± 10 and 45 ± 5 Myr for these clusters. The distances of 2.9 ± 0.1, 4.8 ± 0.4, 3.6 ± 0.1 and 3.8 ± 0.1 kpc determined using parallax are similar to the values derived by the isochrone-fitting method. Mass function slopes are found to be in good agreement with the Salpeter value. The total masses are derived as 348, 595, 763 and 1088 M⊙ for Czernik 14, Haffner 14, Haffner 17 and King 10, respectively. Evidence for the existence of a mass-segregation effect is observed in each cluster. Using the Galactic potential model, Galactic orbits are derived for the clusters. It is found that all four clusters follow a circular path around the Galactic Centre.

The Strikingly Metal-rich Halo of the Sombrero Galaxy*

Abstract The nature of the Sombrero galaxy (M104 = NGC 4594) has remained elusive despite many observational studies at a variety of wavelengths. Here we present Hubble Space Telescope imaging of two fields at ∼16 and 33 kpc along the minor axis to examine stellar metallicity gradients in the extended spheroid. We use this imaging, extending more than 2 mag below the tip of the red giant branch, in combination with artificial star tests to forward model observed color–magnitude diagrams, measuring metallicity distribution functions (MDFs) at different radii along the minor axis. An important and unexpected result is that the halo of the Sombrero is strikingly metal-rich: even the outer field, located at ∼17 effective radii of the bulge, has a median metallicity [Z/H] ∼ −0.15, and the fraction of stars with [Z/H] < −1.0 is negligible. This is unprecedented among massive galaxy halos studied to date, even among giant ellipticals. We find significant radial metallicity gradients, characterized by an increase in the fraction of metal-poor stars with radius and a gradient in median metallicity of ∼0.01 dex kpc−1. The density profile is well fit by power laws with slopes that exhibit a dependence on metallicity, with flatter slopes for more metal-poor stars. We discuss our results in the context of recent stellar MDF studies of other nearby galaxies and potential formation scenarios for the Sombrero galaxy.

Project AMIGA: Distance and Metallicity Gradients along Andromeda’s Giant Southern Stream from the Red Clump∗

Abstract The Giant Southern Stream (GSS) of M31, a keystone signature of a major accretion event, yields crucial constraints on M31 formation and evolution models. Currently, our understanding of the GSS, in terms of both its geometry and its chemistry, results from either wide-field imaging probing only a few magnitudes below the red giant branch tip, or deep imaging or spectroscopy of isolated regions. Here, we take an alternative approach, using Hubble Space Telescope (HST) imaging to characterize the horizontal branch red clump (RC) using unbinned maximum likelihood fits to luminosity functions from observed color–magnitude diagrams. Comparing the RC mean magnitude across three fields at projected distances of 21, 52, and 80 kpc from M31, we find a line-of-sight distance gradient identical to recent literature measurements in fields along the core. We also find tentative evidence that the line-of-sight distance dispersion increases with projected distance from M31. Meanwhile, the metallicity in the 52 kpc field westward of the GSS core is at least as high as that in the 21 kpc GSS core field, and the peak colors of the RC in these two fields imply identical metallicities to within 0.2 dex. We discuss implications for distance and metallicity gradients both along and perpendicular to the GSS in the context of recent ground-based photometric and spectroscopic results, including evidence for a dropoff in metallicity moving westward from the GSS, as well as prospects for further constraining stellar populations in the vicinity of the GSS.

GalMod: A Galactic Synthesis Population Model

Abstract We present a new Galaxy population synthesis Model, GalMod. GalMod is a star-count model featuring an asymmetric bar/bulge as well as spiral arms and related extinction. The model, initially introduced in Pasetto et al., has been here completed with a central bar, a new bulge description, new disk vertical profiles, and several new bolometric corrections. The model can generate synthetic mock catalogs of visible portions of the Milky Way, external galaxies like M31, or N-body simulation initial conditions. At any given time, e.g., at a chosen age of the Galaxy, the model contains a sum of discrete stellar populations, namely the bulge/bar, disk, and halo. These populations are in turn the sum of different components: the disk is the sum of the spiral arms, thin disks, a thick disk, and various gas components, while the halo is the sum of a stellar component, a hot coronal gas, and a dark-matter component. The Galactic potential is computed from these population density profiles and used to generate detailed kinematics by considering up to the first four moments of the collisionless Boltzmann equation. The same density profiles are then used to define the observed color–magnitude diagrams in a user-defined field of view (FoV) from an arbitrary solar location. Several photometric systems have been included and made available online, and no limits on the size of the FoV are imposed thus allowing full-sky simulations, too. Finally, we model the extinction by adopting a dust model with advanced ray-tracing solutions. The model's Web page (and tutorial) can be accessed at www.GalMod.org and support is provided at Galaxy.Model@yahoo.com.

COCOA Code for Creating Mock Observations of Star Cluster Models

We introduce and present results from the COCOA (Cluster simulatiOn Comparison with ObservAtions) code that has been developed to create idealized mock photometric observations using results from numerical simulations of star cluster evolution. COCOA is able to present the output of realistic numerical simulations of star clusters carried out using Monte Carlo or \textit{N}-body codes in a way that is useful for direct comparison with photometric observations. In this paper, we describe the COCOA code and demonstrate its different applications by utilizing globular cluster (GC) models simulated with the MOCCA (MOnte Carlo Cluster simulAtor) code. COCOA is used to synthetically observe these different GC models with optical telescopes, perform PSF photometry and subsequently produce observed colour magnitude diagrams. We also use COCOA to compare the results from synthetic observations of a cluster model that has the same age and metallicity as the Galactic GC NGC 2808 with observations of the same cluster carried out with a 2.2 meter optical telescope. We find that COCOA can effectively simulate realistic observations and recover photometric data. COCOA has numerous scientific applications that maybe be helpful for both theoreticians and observers that work on star clusters. Plans for further improving and developing the code are also discussed in this paper.

Evolution over time of the Milky Way’s disc shape

Context. Galactic structure studies can be used as a path to constrain the scenario of formation and evolution of our Galaxy. The dependence with the age of stellar population parameters would be linked with the history of star formation and dynamical evolution. Aims. We aim to investigate the structures of the outer Galaxy, such as the scale length, disc truncation, warp and flare of the thin disc and study their dependence with age by using 2MASS data and a population synthesis model (the so-called Besançon Galaxy Model). Methods. We have used a genetic algorithm to adjust the parameters on the observed colour–magnitude diagrams at longitudes 80° ≤ ℓ ≤ 280° for | b | ≤ 5.5°. We explored parameter degeneracies and uncertainties. Results. We identify a clear dependence of the thin disc scale length, warp and flare shapes with age. The scale length is found to vary between 3.8 kpc for the youngest to about 2 kpc for the oldest. The warp shows a complex structure, clearly asymmetrical with a node angle changing with age from approximately 165° for old stars to 195° for young stars. The outer disc is also flaring with a scale height that varies by a factor of two between the solar neighbourhood and a Galactocentric distance of 12 kpc. Conclusions. We conclude that the thin disc scale length is in good agreement with the inside-out formation scenario and that the outer disc is not in dynamical equilibrium. The warp deformation with time may provide some clues to its origin.

Globular clusters and the evolution of their multiple stellar populations

AbstractOur knowledge of the formation and early evolution of globular clusters (GCs) has been totally shaken with the discovery of the peculiar chemical properties of their long-lived host stars. Therefore, the interpretation of the observed Colour Magnitude Diagrams (CMD) and of the properties of the GC stellar populations requires the use of new stellar models computed with relevant chemical compositions. In this paper we use the grid of evolution models for low-mass stars computed by Chantereau et al. (2015) with the initial compositions of second-generation stars as predicted by the fast rotating massive stars scenario to build synthesis models of GCs. We discuss the implications of the assumed initial chemical distribution on 13 Gyr isochrones. We build population synthesis models to predict the fraction of stars born with various helium abundances in present day globular clusters (assuming an age of 13 Gyr). With the current assumptions, 61 % of stars on the main sequence are predicted to be born with a helium abundance in mass fraction, Yini, smaller than 0.3 and only 11 % have a Yini larger than 0.4. Along the horizontal branch, the fraction of stars with Yini inferior to 0.3 is similar to that obtained along the main sequence band (63 %), while the fraction of very He-enriched stars is significantly decreased (only 3 % with Yini larger than 0.38).

THE ADVANCED CAMERA FOR SURVEYS NEARBY GALAXY SURVEY TREASURY. V. RADIAL STAR FORMATION HISTORY OF NGC 300

We present new Hubble Space Telescope (HST) observations of NGC 300 taken as part of the Advanced Camera for Surveys Nearby Galaxy Survey Treasury (ANGST). Individual stars are resolved in these images down to an absolute magnitude of MF814W = 1.0 (below the red clump). We determine the star formation history of the galaxy in six radial bins by comparing our observed color–magnitude diagrams (CMDs) with synthetic CMDs based on theoretical isochrones. We find that the stellar disk out to 5.4 kpc is primarily old, in contrast with the outwardly similar galaxy M33. We determine the scale length as a function of age and find evidence for inside–out growth of the stellar disk: the scale length has increased from 1.1 ± 0.1 kpc 10 Gyr ago to 1.3 ± 0.1 kpc at present, indicating a buildup in the fraction of young stars at larger radii. As the scale length of M33 has recently been shown to have increased much more dramatically with time, our results demonstrate that two galaxies with similar sizes and morphologies can have very different histories. With an N-body simulation of a galaxy designed to be similar to NGC 300, we determine that the effects of radial migration should be minimal. We trace the metallicity gradient as a function of time and find a present-day metallicity gradient consistent with that seen in previous studies. Consistent results are obtained from archival images covering the same radial extent but differing in placement and filter combination.

Chemical evolution models for the dwarf spheroidal galaxies Leo 1 and Leo 2

Aims. We investigate the chemical evolutionary history of the dwarf spheroidal galaxies Leo 1 and Leo 2 by means of predictions from a detailed chemical evolution model compared to observations. The model adopts up to date nucleosynthesis and takes into account the role played by supernovae of different types (Ia, II), allowing us to follow in detail the evolution of several chemical elements (H, D, He, C, N, O, Mg, Si, S, Ca, Fe, Ba, and Eu). Methods. Each galaxy model is specified by the prescriptions of the star formation rate and by the galactic wind efficiency chosen to reproduce the main features of these galaxies, in particular the stellar metallicity distributions and several abundance ratios. These parameters are constrained by the star formation histories of the galaxies as inferred by the observed color–magnitude diagrams, indicating extended star formation episodes occurring at early epochs, but also with hints of intermediate stellar populations. Results. The main observed features of the galaxies Leo 1 and Leo 2 can be very well explained by chemical evolution models according to the following scenarios: the star formation occurred in two long episodes at 14 Gyr and 9 Gyr ago that lasted 5 and 7 Gyr, respectively, with a low efficiency (ν = 0. 6G yr −1 ) in Leo 1, whereas the star formation history in Leo 2 is characterized by one episode at 14 Gyr ago that lasted 7 Gyr, also with a low efficiency (ν = 0. 3G yr −1 ). In both galaxies an intense wind (nine and eight times the star formation rate – wi = 9 and 8 in Leo 1 and Leo 2, respectively) takes place which defines the pattern of the abundance ratios and the shape of the stellar metallicity distribution at intermediate to high metallicities. Conclusions. The observational constraints can only be reproduced with the assumption of gas removal by galactic winds.

Chemical analysis of carbon stars in the Local Group

Aims. We present new results of our ongoing chemical study of carbon stars in Local Group galaxies to test the critical dependence of s-process nucleosynthesis on the stellar metallicity. Methods. We collected optical spectra with the VLT/UVES instrument of two carbon stars found in the Carina Dwarf Spheroidal (dSph) galaxy, namely ALW-C6 and ALW-C7. We performed a full chemical analysis using the new generation of hydrostatic, spherically symmetric carbon-rich model atmospheres and the spectral synthesis method in LTE. Results. The luminosities, atmosphere parameters and chemical composition of ALW-C6 and ALW-C7 are compatible with these stars being in the TP-AGB phase undergoing third dredge-up episodes, although their extrinsic nature (external pollution in a binary stellar system) cannot be definitively excluded. Our chemical analysis shows that the metallicity of both stars agree with the average metallicity ([Fe/H] ∼− 1.8 dex) previously derived for this satellite galaxy from the analysis of both low resolution spectra of RGB stars and the observed colour magnitude diagrams. ALW-C6 and ALW-C7 present strong s-element enhancements, [s/Fe] =+ 1.6, +1.5, respectively. These enhancements and the derived s-process indexes [ls/Fe], [hs/Fe] and [hs/ls] are compatible with theoretical s-process nucleosynthesis predictions in low mass AGB stars (∼1.5 M� ) on the basis that the 13 C(α,n) 16 O is the main source of neutrons. Furthermore, the analysis of C2 and CN bands reveals a large carbon enhancement (C/O ∼ 7 and 5, respectively), much larger than the values typically found in galactic AGB carbon stars (C/O ∼ 1−2). This is also in agreement with the theoretical prediction that AGB carbon stars are formed more easily through third dredge-up episodes as the initial stellar metallicity drops. However, theoretical low-mass AGB models apparently fail to simultaneously fit the observed s-element and carbon enhancements. On the other hand, Zr is found to be less enhanced in ALW-C7 compared to the other elements belonging to the same s-peak. Although the abundance errors are large, the fact that in this star the abundance of Ti (which has a similar condensation temperature to Zr) seems also to be lower than those of others metals, may indicate the existence of some depletion into dust-grains in its photosphere.

Synthetic Hertzsprung—Russell diagrams of low-mass stars and the efficiency of colour transformation in determining the open cluster ages

The scales of the effective temperature T eff and bolometric correction (BC) are necessary for isochrones to be compared with observed colour—magnitude diagrams (CMDs) of star clusters. Reliable scales can be found, providing improved numerical relations between observed and theoretical quantities. A detailed analysis is presented of the effects which the log T eff, B−V and BC scales have on the transformation of evolutionary tracks and time isochrones from the theoretical Hertzsprung—Russell diagram to the CMD. We present a comparison between different scales of published transformations to find the ages of some selected open clusters.

A new infrared view of evolved stars in IZw 18

We report results from near-infrared imaging of IZw 18, the most metal-poor galaxy in the local universe, with NICMOS on board the Hubble Space Telescope (HST). Observations were obtained in the broad F205W filter (~K) and in the medium-wide bands F171M and F180M, the latter which cover strong molecular absorption features in cool stars. The new data, together with previously obtained HST/NICMOS images in the F110W (~J) and F160W (~H) filters, provide a census of the cool stellar population in this chemically unevolved galaxy. We find that stars as old as ~1 Gyr are required to explain the observed colour–magnitude diagrams. Combining broad and medium-wide band photometry, we have classified the observed stars. The observed stellar populations, to the depth of our data, are dominated by luminous red supergiant stars. However, an intermediate-age component is also present. We have identified carbon star candidates, and show that they dominate the stellar content for intermediate mass and age. We show that low metallicity intermediate-age stars have, for a given colour, a F110W (~J-band) excess compared to local carbon stars. The carbon stars identified in IZw 18 are the most distant (more than 10 Mpc) resolved examples yet discovered.

Is I ZW 18 Really a Young Galaxy

To understand the role of dwarfs in the context of galaxy formation and evolution, we are studying the star-formation history of some representative irregular and blue compact dwarf galaxies by comparing the observed colour–magnitude diagrams with synthetic ones based on homogeneous sets of stellar evolutionary tracks. Here we present the results obtained for the blue compact I Zw 18, the most metal-poor galaxy known. Our simulations suggest star-formation activity started around 1–0.3 Gyr ago, with evidence of an intense burst around 15–20 Myr ago. I Zw 18 has turned out not to be a young object as previously suggested.

Star formation in dwarf irregular galaxies NGC 6822

view Abstract Citations (75) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Star Formation in Dwarf Irregular Galaxies: NGC 6822 Marconi, G. ; Tosi, M. ; Greggio, L. ; Focardi, P. Abstract Our proposed method to study the star-formation histories in nearby irregulars is here applied to NGC 6822. To this purpose we have obtained accurate CCD photometry of three regions in the galaxy, reaching V~23.5 with the required accuracy of σ_BVR_ <= 0.1 mag. Major information on the stellar populations, star-formation rates, and initial mass functions of these regions in the last 1 Gyr are derived from the comparison of the observational color-magnitude diagrams and luminosity functions with the corresponding synthetic diagrams and luminosity functions generated by a numerical simulation code based on stellar evolutionary tracks. We find that in the last 1 Gyr the star-formation activity has been rather continuous, possibly occurring in long episodes of moderate activity separated by short quiescent periods, and that the initial mass function exponent α is in the vicinity of Salpeter `s value, i.e., slightly flatter than in the solar neighborhood. The studied regions of NGC 6822 appear to contain different stellar populations. The preferred star formation regime for region A is a continuous activity coupled with a moderately flat α. For region C, instead, our best models suggest a discontinuous star formation and a moderately steep α. NGC 6822 is the last object of our original sample of five nearby irregulars. We can then draw the scenario resulting from this sample: most of the observed fields appear to have experienced a gasping regime of star formation, with average exponent α =2.35 for the initial mass function. Despite their small sizes, the majority of the sample galaxies contain different stellar populations in different regions. Publication: The Astronomical Journal Pub Date: January 1995 DOI: 10.1086/117265 arXiv: arXiv:astro-ph/9409013 Bibcode: 1995AJ....109..173M Keywords: Dwarf Galaxies; Histories; Irregular Galaxies; Star Formation; Astronomical Photometry; Charge Coupled Devices; Color-Magnitude Diagram; Computerized Simulation; Galactic Evolution; Star Formation Rate; Stellar Luminosity; Astronomy; GALAXIES: INDIVIDUAL: NGC 6822; GALAXIES: IRREGULAR; STARS: FORMATION; GALAXIES: STELLAR CONTENT; Astrophysics E-Print: plain Tex format full text sources arXiv | ADS | data products SIMBAD (1781) NED (5) CDS (1)

Star formation in dwarf irregular galaxies - DDO 210 and NGC 3109

The method proposed in Paper I [Tosi et al., AJ, 102, 951 (1991)] to study the star formation histories in nearby irregulars is here applied to the galaxies DDO 210 and NGC 3109. To this purpose we have obtained deep CCD photometry of two regions in DDO 210 and three regions in NGC 3109, reaching V ≃ 24 with the required accuracy of σ v ≤ 0.1 mag. Major information on the stellar populations, star formation rates, and initial mass functions of these regions in the last 1 Gyr are derived from the comparison of the observational color-magnitude diagrams and luminosity functions, with the corresponding synthetic diagrams and luminosity functions generated by a numerical simulation code based on stellar evolutionary tracks

Comparisons between observational color-magnitude diagrams and synthetic cluster diagrams for young star clusters in the Magellanic Clouds

Young star clusters (&lt;3 × 108 yr) in the Maqellanic Clouds (MC) can be used to test the current status of the theory of stellar evolution as applied to intermediate and massive stars. The color-magnitude diagram of many young clusters in the MC shows, unlike the case of clusters in our Galaxy, large numbers of stars in both the main sequence and post main sequence evolutionary phases. Usina a arid of stellar evolution models, synthetic cluster H-R diagrams are constructed and compared to observed color-magnitude diagrams to determine the age, age spread, and composition for any given cluster. In addition, for those cases where the data is of high quality, detailed comparisons between theory and observation can provide a diagnostic of the accuracy of the stellar evolution models. Initial indications of these comparisons suggest that the theoretical models should be altered to include: a larger value for the mixing length parameter (α), a larger rate of mass loss during the asymptotic giant branch (AGB) phase, and possibly convective overshoot during the core burning phases.