Color-magnitude Diagrams Of Stars Research Articles

Stellar progenitors of black holes: insights from optical and infrared observations

AbstractHere are reviewed the insights from observations at optical and infrared wavelengths for low mass limits above which stars do not seem to end as luminous supernovae. These insights are: (1) the absence in archived images of nearby galaxies of stellar progenitors of core-collapse supernovae above 16-18 M⊙, (2) the identification of luminous-massive stars that quietly disappear without optically bright supernovae, (3) the absence in the nebular spectra of supernovae of type II-P of the nucleosynthetic products expected from progenitors above 20 M⊙, (4) the absence in color magnitude diagrams of stars in the environment of historic core-collapse supernovae of stars with ⩾20 M⊙. From the results in these different areas of observational astrophysics, and the recently confirmed dependence of black hole formation on metallicity and redshift of progenitors, it is concluded that a large fraction of massive stellar binaries in the universe end as binary black holes.

ON THE INCORPORATION OF METALLICITY DATA INTO MEASUREMENTS OF STAR FORMATION HISTORY FROM RESOLVED STELLAR POPULATIONS

ABSTRACT The combination of spectroscopic stellar metallicities and resolved star color–magnitude diagrams (CMDs) has the potential to constrain the entire star formation history (SFH) of a galaxy better than fitting CMDs alone (as is most common in SFH studies using resolved stellar populations). In this paper, two approaches to incorporating external metallicity information into CMD-fitting techniques are presented. Overall, the joint fitting of metallicity and CMD information can increase the precision of measured age–metallicity relationships (AMRs) and star formation rates by 10% over CMD fitting alone. However, systematics in stellar isochrones and mismatches between spectroscopic and photometric determinations of metallicity can reduce the accuracy of the recovered SFHs. I present a simple mitigation of these systematics that can reduce their amplitude to the level obtained from CMD fitting alone, while ensuring that the AMR is consistent with spectroscopic metallicities. As is the case in CMD-fitting analysis, improved stellar models and calibrations between spectroscopic and photometric metallicities are currently the primary impediment to gains in SFH precision from jointly fitting stellar metallicities and CMDs.

A GLOBAL STAR-FORMING EPISODE IN M31 2–4 GYR AGO

We have identified a major global enhancement of star formation in the inner M31 disk that occurred between 2-4 Gyr ago, producing $\sim$60% of the stellar mass formed in the past 5 Gyr. The presence of this episode in the inner disk was discovered by modeling the optical resolved star color-magnitude diagrams of low extinction regions in the main disk of M31 (3$<$R$<$20 kpc) as part of the Panchromatic Hubble Andromeda Treasury. This measurement confirms and extends recent measurements of a widespread star formation enhancement of similar age in the outer disk, suggesting that this burst was both massive and global. Following the galaxy-wide burst, the star formation rate of M31 has significantly declined. We briefly discuss possible causes for these features of the M31 evolutionary history, including interactions with M32, M33 and/or a merger.

CCD SDSS gr Photometry of Poorly Studied Star Clusters in the Large Magellanic Cloud

We present for the first time CCD SDSS gr photometry, obtained at the Gemini South telescope with the GMOS attached, of stars in the field of the poorly studied star clusters NGC1768, HS 85, SL 676, NGC2107, NGC2190, and SL 866, which are distributed in the main body of the Large Magellanic Cloud. We applied a subtraction procedure to statistically clean the cluster CMDs from field star contamination. In order to disentangle cluster features from those belonging to their surrounding fields, we applied a subtraction procedure which makes use of variable cells to reproduce the field star color-magnitude diagrams (CMDs) as closely as possible. We then traced their stellar density radial profiles from star counts performed over the cleaned field stars dataset and derived their radii. Using the cleaned cluster CMDs, we estimated ages and metallicities from matching theoretical isochrones computed for the SDSS system. The studied star clusters have ages from 0.1 up to 2.0 Gyr and are of slightly metal-poor metal content ([Fe/H]≈−0.4 dex).

MEASURING GALAXY STAR FORMATION RATES FROM INTEGRATED PHOTOMETRY: INSIGHTS FROM COLOR-MAGNITUDE DIAGRAMS OF RESOLVED STARS

We use empirical star formation histories (SFHs), measured from HST-based resolved star color-magnitude diagrams, as input into population synthesis codes to model the broadband spectral energy distributions (SEDs) of ~50 nearby dwarf galaxies (6.5 < log M/M_* < 8.5, with metallicities ~10% solar). In the presence of realistic SFHs, we compare the modeled and observed SEDs from the ultraviolet (UV) through near-infrared (NIR) and assess the reliability of widely used UV-based star formation rate (SFR) indicators. In the FUV through i bands, we find that the observed and modeled SEDs are in excellent agreement. In the Spitzer 3.6micron and 4.5micron bands, we find that modeled SEDs systematically over-predict observed luminosities by up to ~0.2 dex, depending on treatment of the TP-AGB stars in the synthesis models. We assess the reliability of UV luminosity as a SFR indicator, in light of independently constrained SFHs. We find that fluctuations in the SFHs alone can cause factor of ~2 variations in the UV luminosities relative to the assumption of a constant SFH over the past 100 Myr. These variations are not strongly correlated with UV-optical colors, implying that correcting UV-based SFRs for the effects of realistic SFHs is difficult using only the broadband SED. Additionally, for this diverse sample of galaxies, we find that stars older than 100 Myr can contribute from <5% to100% of the present day UV luminosity, highlighting the challenges in defining a characteristic star formation timescale associated with UV emission. We do find a relationship between UV emission timescale and broadband UV-optical color, though it is different than predictions based on exponentially declining SFH models. Our findings have significant implications for the comparison of UV-based SFRs across low-metallicity populations with diverse SFHs.

HST-ACS photometry of the isolated dwarf galaxy VV124=UGC 4879

We present deep V and I photometry of the isolated dwarf galaxy VV124=UGC 4879, obtained from archival images taken with the Hubble Space Telescope – Advanced Camera for Surveys. In the color-magnitude diagrams of stars at distances greater than 40′′ from the center of the galaxy, we clearly identify a well-populated, old horizontal branch (HB) for the first time. We show that the distribution of these stars is more extended than that of red clump stars. This implies that very old and metal-poor populations dominate in the outskirts of VV124. We also identify a massive (M = 1.2 ± 0.2 × 104 M ) young (age = 250 ± 50 Myr) star cluster (C1), as well as another of younger age (C2, <∼30 ± 10 Myr) with a mass similar to classical open clusters (M ≤ 3.3 ± 0.5 × 103 M ). Both clusters lie at projected distances less than 100 pc from the center of the galaxy.

AN EXTENDED STAR CLUSTER AT THE OUTER EDGE OF THE SPIRAL GALAXY M 33

We report the discovery of an extended globular-like star cluster, M 33-EC1, at the outer edge of the spiral galaxy M 33. The distance to the cluster is 890 kpc, and it lies at a projected distance of 12.5 kpc from the center of M 33. Old age (7 Gyr) and low metallicity ([M/H] –1.4) are estimated on the basis of isochrone fits. Color-magnitude diagrams of stars, located in the cluster's area, and photometric and structural parameters of the cluster are presented. The cluster's luminosity (MV = –6.6) and half-light radius (r h = 20.3 pc) are comparable to those of the extended globular clusters, discovered in more luminous Local Group galaxies, the Milky Way and M 31. Extended globular clusters are suspected to be remnants of accreted dwarf galaxies, and the finding of such a cluster in the late-type dwarf spiral galaxy M 33 would imply a complex merging history in the past.

Chronicling the histories of galaxies at distances of 1 to 20 Mpc: simulated performance of 30, 50, and 100m telescopes

A key science driver for future ELTs is to chronicle the complete formation and evolutionary histories of a meaningful number of nearby galaxies through their resolved stars. The goal will be to measure the entire star formation and chemical enrichment histories of a sample of galaxies that includes all Hubble types and covers all of their components, demanding photometry of stars in regions with high surface brightness at distances of up to 20Mpc. We present simulations that compare the abilities of 20, 30, 50, and 100m telescopes to recover the correct stellar population mix represented in field star color-magnitude diagrams observed with $J$ , $H$ , and $K$ filters. As input, our simulations use scenes containing stars drawn from a mix of model isochrones with differing ages and metallicities, with surface densities set to match that found in the M31 bulge and at the effective radius of NGC 3379. We convolve these scenes with PSFs corresponding to the projected performance of MCAO systems containing two deformable mirrors, including the effect of realistic variations in the atmospheric turbulence profile over the span of the observations. These simulations provide a way to evaluate the scientific advances enabled by ELTs of differing apertures in the area of extragalactic stellar populations.

Strömgren Photometry of Field Red Giants in the LMC

We present here preliminary results of our program to obtain rough abundance estimates for &gt; 104 red giant branch and red clump stars in four fields of the LMC. The derived abundance distributions will be a strong constraint on models of the LMC's star-formation history based on field star color-magnitude diagrams.