Intermediate-age Stars Research Articles

The True Luminosities of Planetary Nebulae in M31's Bulge: Massive Central Stars from an Old Stellar Population

Abstract We measure the Balmer decrements of 23 of the brightest planetary nebulae (PNe) in the inner bulge (r ≲ 3′) of M31 and deredden the bright end of the region’s [O iii] λ5007 PN luminosity function. We show that the most luminous PNe produce ≳1200 L ⊙ of power in their [O iii] λ5007 line, implying central star luminosities of at least ∼11,000 L ⊙. Even with the most recent accelerated-evolution post-AGB models, such luminosities require central star masses in excess of 0.66 M ⊙ and main-sequence progenitors of at least ∼2.5 M ⊙. Since M31's bulge has very few intermediate-age stars, we conclude that conventional single-star evolution cannot be responsible for these extremely luminous objects. We also present the circumstellar extinctions for the region’s bright PNe and demonstrate that the distribution is similar to that found for PNe in the Large Magellanic Cloud, with a median value of A 5007 = 0.71. Finally, we compare our results to extinction measurements made for PNe in the E6 elliptical NGC 4697 and the interacting lenticular NGC 5128. We show that such extinctions are not unusual and that the existence of very high-mass PN central stars is a general feature of old stellar populations. Our results suggest that single-star population synthesis models significantly underestimate the maximum luminosities and total integrated light of AGB stars.

What do planetary nebulae and H II regions reveal about the chemical evolution of nearby dwarf galaxies?

AbstractThe Local Group contains a great number of dwarf irregulars and spheroidals, for which the spectroscopy of individual stars can be obtained. Thus, the chemical evolution of these galaxies can be traced, with the only need of finding populations spanning a large age range and such that we can accurately derive the composition. Planetary nebulae (PNe) are old- and intermediate-age star remnants and their chemical abundances can be obtained up to 3-4 Mpc. H ii regions, which are brighter and much easily detected, represent galaxies young content. PNe and H ii regions share similar spectroscopic features and are analysed in the same way. Both are among the best tracers of the chemical evolution allowing to draw the chemical time line of nearby galaxies. The focus in this review are the PN and H ii region populations as constraints to the chemical evolution models and the mass-metallicity relation of the local universe.

Cluster kinematics and stellar rotation in NGC 419 with MUSE and adaptive optics

We present adaptive optics (AO) assisted integral-field spectroscopy of the intermediate-age star cluster NGC 419 in the Small Magellanic Cloud. By investigating the cluster dynamics and the rotation properties of main sequence turn-off stars (MSTO), we demonstrate the power of AO-fed MUSE observations for this class of objects. Based on 1 049 radial velocity measurements, we determine a dynamical cluster mass of 1.4+/-0.2x10^5 M_sun and a dynamical mass-to-light ratio of 0.67+/-0.08, marginally higher than simple stellar population predictions for a Kroupa initial mass function. A stacking analysis of spectra at both sides of the extended MSTO reveals significant rotational broadening. Our analysis further provides tentative evidence that red MSTO stars rotate faster than their blue counterparts. We find average V sin i values of 87+/-16 km/s and 130+/-22 km/s for blue and red MSTO stars, respectively. Potential systematic effects due to the low spectral resolution of MUSE can reach 30 km/s but the difference in V sin i between the populations is unlikely to be affected.

The Andromeda galaxy’s most important merger about 2 billion years ago as M32’s likely progenitor

Although the proximity of the Andromeda galaxy (M31) offers an opportunity to understand how mergers affect galaxies, uncertainty remains about M31's most important mergers. Previous studies focused individually on the giant stellar stream or the impact of M32 on M31's disk, thereby suggesting many substantial satellite interactions. Yet models of M31's disk heating and the similarity between the stellar populations of different tidal substructures in M31's outskirts both suggested a single large merger. M31's stellar halo (its outer low-surface-brightness regions) is built up from the tidal debris of satellites and provides information about its important mergers. Here we use cosmological models of galaxy formation to show that M31's massive and metal-rich stellar halo, containing intermediate-age stars, dramatically narrows the range of allowed interactions, requiring a single dominant merger with a large galaxy (with stellar mass about 2.5 x 10^10 solar masses, the third largest member of the Local Group) about 2 Gyr ago. This single event explains many observations that were previously considered separately: M31's compact and metal-rich satellite M32 is likely to be the stripped core of the disrupted galaxy, its rotating inner stellar halo contains most of the merger debris, and the giant stellar stream is likely to have been thrown out during the merger. This interaction may explain M31's global burst of star formation about 2 Gyr ago in which approximately a fifth of its stars were formed. Moreover, M31's disk and bulge were already in place, suggesting that mergers of this magnitude need not dramatically affect galaxy structure.

Photometric study of the SMCNOD using variable stars from the OGLE-IV survey

We present a study of a recently discovered stellar overdensity near the northern edge of the Small Magellanic Cloud (SMCNOD). We exploited variable stars from the fourth release of the Optical Gravitational Lensing Experiment (OGLE). We used mainly pulsating variable stars and investigated their potential association with the SMCNOD using their spatial distribution and distances. We found four rather spatially concentrated anomalous Cepheids and eight evenly dispersed RR~Lyrae stars to be most likely members of this overdensity. The anomalous Cepheids inside the SMCNOD trace possible intermediate-age population with ages ranging between $2-4.5$ Gyr. The age distribution of anomalous Cepheids seems to be in a good agreement with the age distribution of anomalous Cepheids in the SMC. Using empirical relations for RR~Lyrae stars we determined the median metallicity for a possible old population in the SMCNOD to be $\rm [Fe/H]_{\rm SMCNOD}=-1.71\pm0.21$ dex, which is in agreement with median metallicity of the old SMC population. The density profile for anomalous Cepheids shows a small anomaly at the position of the SMCNOD, on the other hand, RR~Lyrae variables show no such deviation. The probability of finding the observed number of variable stars at the location of the SMCNOD by chance is very low for anomalous Cepheids (0.7 %) but high for RR~Lyrae stars (13.0 %). Based on its variable stars content, we thus confirm the presence of a modest overdensity in intermediate-age stars in the SMCNOD and conclude that it probably has its origin in the SMC rather than to be the remnant of an accreted dwarf galaxy.

SiO Masers in the Galactic Bulge and Disk: Kinematics from the BAaDE Survey

Abstract We present the first results from the Bulge Asymmetries and Dynamic Evolution (BAaDE) survey. Though only a subset of the complete survey (∼2700 out of ∼20,000 final sources), our data comprise the largest radio kinematic survey to date of stellar SiO masers observed toward the Galactic bulge and plane from −15° < l < +12° and −6° < b < +6°. Our sources include a substantial number of line-of-sight velocities in high extinction regions within ±1° of the Galactic plane. When matched with 2MASS photometry, our radio-detected sample lies significantly brighter than and redward of the first red giant branch tip, reaching extremes of (J − Ks)0 > 8, colors consistent with Mira variables and mass-losing AGB stars. We see a clean division into two kinematic populations: a kinematically cold (σ ∼ 50 km s−1) population that we propose is in the foreground disk, consisting of giants with 2MASS Ks < 5.5, and a kinematically hot (σ ∼ 100 km s−1) candidate bulge/bar population for most giants with Ks > 5.5. Only the kinematically hot giants with Ks > 5.5 include the reddest stars. Adopting 8.3 kpc to the Galactic center, and correcting for foreground extinction, we find that most of the sources have M bol ∼ −5, consistent with their being luminous and possibly intermediate-age AGB stars. We note some tension between the possible intermediate age of the kinematically hot population, and its high-velocity dispersion compared to the disk.

The Effects of Binary Stars on the Color–Magnitude Diagrams of Young-age Massive Star Clusters

Abstract Extended main-sequence turnoffs (eMSTO) have been observed in the color–magnitude diagram (CMD) of intermediate-age and young star clusters. The origin of the eMSTO phenomenon is still highly debated. Calculations show that the blue and faint (BF) stars in the CMD of NGC 1866 are hydrogen main-sequence (MS) + naked He-star systems. The He star derives from the massive star of a binary system. The BF stars and the red and faint MSTO stars belong to the same stellar population. The values of m F336W and m F336W −m F814W of the BF stars are mainly determined by the masses of He stars and H-MS stars, respectively. The behaviors of the BF stars in the CMD are well explained by the H-MS+He-star systems. The BF stars provide a strict restriction on the age of the stellar population. Moreover, the bimodal MS of NGC 1866 can also be reproduced by a younger binary population. The calculations show that part of the blue and bright (BB) MS stars of NGC 1866 are H-MS+He-star systems, H-MS+white dwarf systems, and merged stars in a binary scenario. The H-MS stars of the H-MS+He-star systems for the BB stars are significantly more massive than those of the BF stars. Once the H-MS+He-star systems and their membership in NGC 1866 are confirmed, the extended star formation histories and the effects of binaries can be confirmed in the young star cluster.

The star formation history in the M31 bulge

We present the study of stellar populations in the central 5.5' (~1.2 kpc) of the M31 bulge by using the optical color magnitude diagram derived from HST ACS WFC/HRC observations. In order to enhance image quality and then obtain deeper photometry, we construct Nyquist-sampled images and use a deconvolution method to detect sources and measure their photometry. We demonstrate that our method performs better than DOLPHOT in the extremely crowded region. The resolved stars in the M31 bulge have been divided into nine annuli and the color magnitude diagram fitting is performed for each of them. We confirm that the majority of stars (> 70%) in the M31 bulge are indeed very old (>5 Gyr) and metal-rich ([Fe/H] > 0.3). At later times, the star formation rate decreased and then experienced a significant rise around 1 Gyr ago, which pervaded the entire M31 bulge. After that, stars formed at less than 500 Myr ago in the central 130". Through simulation, we find that these intermediate-age stars cannot be the artifacts introduced by the blending effect. Our results suggest that although the majority of the M31 bulge are very old, the secular evolutionary process still continuously builds up the M31 bulge slowly. We compare our star formation history with an older analysis derived from the spectral energy distribution fitting, which suggests that the latter one is still a reasonable tool for the study of stellar populations in remote galaxies.

Seeing Red in NGC 1978, NGC 55, and NGC 3109* †

Abstract Spectra of the intermediate-age star cluster NGC 1978 and the dwarf irregular galaxies NGC 55 and NGC 3109 are discussed. The spectra were recorded with the Gemini Multi-object Spectrograph on Gemini South and span the 0.7–1.1 μm wavelength interval. Five slit pointings were observed in NGC 1978, and these are used to examine stochastic effects on the integrated red light from an intermediate-age cluster. The removal of either the brightest M giant or the brightest C star from the co-added spectrum has minor effects on the equivalent withs of the Ca triplet. The most robust signature of C stars in the integrated cluster spectrum at these wavelengths is the CN band head near 7900 Å. The equivalent widths of Ca triplet lines in the NGC 1978 spectrum and in the spectra of individual cluster stars are larger than expected for a scaled-solar abundance system. It is suggested that these stars have a lower than expected surface gravity, which might occur if the stars in NGC 1978 have been subject to extra mixing processes, as suggested by Lederer et al. The near-infrared color profile of NGC 1978 is shown to contain a prominent red cusp in the central 10 arcsec, and the suppression of light from this cusp does not affect the depth of the Ca lines in the integrated spectrum. The NGC 55 spectra run parallel to the major axis, and a gradient is found in the strength of the Ca lines, in the sense that the Ca lines weaken with increasing distance from the disk plane. Comparisons with models suggest that the disk light is dominated by stars with ages 1–2 Gyr, in agreement with star-forming histories (SFHs) obtained from the analysis of color–magnitude diagrams (CMDs). The NGC 55 spectra also sample a large star-forming complex. The age of this complex inferred from comparisons with models is broadly consistent with that estimated from a near-infrared CMD of the same region. The CN band head at 7900 Å in this part of NGC 55 is detected, but this is likely a signature of red supergiants (RSGs) rather than C stars. The NGC 3109 observations sample three different parts of that galaxy but have a low signal-to-noise ratio. Comparisons with models suggest that the light from the NGC 3109 disk at red wavelengths is dominated by RSGs with ages of at most a few tens of Myr, in qualitative agreement with SFHs that are based on photometric measurements.

Not-so-simple stellar populations in nearby, resolved massive star clusters

Around the turn of the last century, star clusters of all kinds were considered ‘simple’ stellar populations. Over the past decade, this situation has changed dramatically. At the same time, star clusters are among the brightest stellar population components and, as such, they are visible out to much greater distances than individual stars, even the brightest, so that understanding the intricacies of star cluster composition and their evolution is imperative for understanding stellar populations and the evolution of galaxies as a whole. In this review of where the field has moved to in recent years, we place particular emphasis on the properties and importance of binary systems, the effects of rapid stellar rotation, and the presence of multiple populations in Magellanic Cloud star clusters across the full age range. Our most recent results imply a reverse paradigm shift, back to the old simple stellar population picture for at least some intermediate-age (∼1–3 Gyr old) star clusters, opening up exciting avenues for future research efforts.

Extended Main-sequence Turn-offs in Intermediate-age Star Clusters: Stellar Rotation Diminishes, but Does Not Eliminate, Age Spreads

Abstract Extended main-sequence turn-off (eMSTO) regions are a common feature in color–magnitude diagrams of young- and intermediate-age star clusters in the Magellanic Clouds. The nature of eMSTOs remains debated in the literature. The currently most popular scenarios are extended star formation activity and ranges of stellar rotation rates. Here we study details of differences in main-sequence turn-off (MSTO) morphology expected from spreads in age versus spreads in rotation rates, using Monte Carlo simulations with the Geneva syclist isochrone models that include the effects of stellar rotation. We confirm a recent finding of Niederhofer et al. that a distribution of stellar rotation velocities yields an MSTO extent that is proportional to the cluster age, as observed. However, we find that stellar rotation yields MSTO crosscut widths that are generally smaller than observed ones at a given age. We compare the simulations with high-quality Hubble Space Telescope data of NGC 1987 and NGC 2249, which are the two only relatively massive star clusters with an age of ∼1 Gyr for which such data is available. We find that the distribution of stars across the eMSTOs of these clusters cannot be explained solely by a distribution of stellar rotation velocities, unless the orientations of rapidly rotating stars are heavily biased toward an equator-on configuration. Under the assumption of random viewing angles, stellar rotation can account for ∼60% and ∼40% of the observed FWHM widths of the eMSTOs of NGC 1987 and NGC 2249, respectively. In contrast, a combination of distributions of stellar rotation velocities and stellar ages fits the observed eMSTO morphologies very well.

Discovery of carbon-rich Miras in the Galactic bulge

Only one carbon-rich (C-rich, hereinafter) Mira variable has so far been suggested as a member of the Galactic bulge and this is in a symbiotic system. Here we describe a method for selecting C-rich candidates from an infrared colour-colour diagram, (J-Ks) vs ([9]-[18]). Follow-up low-resolution spectroscopy resulted in the detection of 8 C-rich Mira variables from a sample of36 candidates towards the Galactic bulge. Our near-infrared photometry indicates that two of these, including the known symbiotic, are closer than the main body of the bulge while a third is a known foreground object. Of the 5 bulge members, one shows He I and [O II] emission and is possibly another symbiotic star. Our method is useful for identifying rare C-rich stars in the Galactic bulge and elsewhere. The age of these C-rich stars and the evolutionary process which produced them remain uncertain. They could be old and the products of either binary mass transfer or mergers, i.e. the descendants of blue stragglers, but we cannot rule out the possibility that they belong to a small in-situ population of metal-poor intermediate age (less than 5 Gyr) stars in the bulge or that they have been accreted from a dwarf galaxy.

The structure of Andromeda II dwarf spheroidal galaxy

We analyze in detail the spatial distribution and kinematic properties of two different stellar populations in Andromeda II (And II) dwarf spheroidal galaxy. We obtained their detailed surface density maps, together with their radial density profiles. The two populations differ not only in age and metallicity, but also in their spatial distribution and kinematics. Old stars ($\gtrsim 11$ Gyr) follow a round distribution well fitted by truncated density profiles. These stars rotate around the projected optical major axis of the galaxy with line-of-sight velocities $v_{los}(r_h) = 16 \pm 3$ km s$^{-1}$ and a velocity gradient of $2.06 \pm 0.21$ km s$^{-1}$ arcmin$^{-1}$. Intermediate-age stars ($\lesssim 9$ Gyr) concentrate in the centre of the galaxy and form an elongated structure extending along the projected optical major axis. This structure appears to rotate with a velocity gradient of $2.24 \pm 0.22$ km s$^{-1}$ arcmin$^{-1}$, and around the optical minor axis. The centres of rotation and kinetic position angles (PA$_{\rm kin}$) of both populations differ. For intermediate-age stars we obtained PA$_{\rm kin} = 18^\circ \pm 2^\circ$ and for the old ones PA$_{\rm kin} = 63^\circ \pm 3^\circ$ in good agreement with photometric PA measured from isopleths fitted to the photometry. We conclude that the two stellar populations may not be fully relaxed and thus be the result of a merger occurred at redshift $z\sim 1.75$.

Disentangling the near-infrared continuum spectral components of the inner 500 pc of Mrk 573: two-dimensional maps

We present a near infrared study of the spectral components of the continuum in the inner 500$\times$500 pc$^2$ of the nearby Seyfert galaxy Mrk573 using adaptive optics near-infrared integral field spectroscopy with the instrument NIFS of the Gemini North Telescope at a spatial resolution of $\sim$50 pc. We performed spectral synthesis using the {\sc starlight} code and constructed maps for the contributions of different age components of the stellar population: young ($age\leq100$ Myr), young-intermediate ($100<age\leq700$ Myr), intermediate-old ($700$ Myr $<age\leq2$ Gyr) and old ($age>2$ Gyr) to the near-IR K-band continuum, as well as their contribution to the total stellar mass. We found that the old stellar population is dominant within the inner 250 pc, while the intermediate age components dominate the continuum at larger distances. A young stellar component contributes up to $\sim$20% within the inner $\sim$70 pc, while hot dust emission and featureless continuum components are also necessary to fit the nuclear spectrum, contributing up to 20% of the K-band flux there. The radial distribution of the different age components in the inner kiloparsec of Mrk573 is similar to those obtained by our group for the Seyfert galaxies Mrk1066, Mrk1157 and NGC1068 in previous works using a similar methodology. Young stellar populations ($\leq$100 Myr) are seen in the inner 200-300 pc for all galaxies contributing with $\ge$20% of the K-band flux, while the near-IR continuum is dominated by the contribution of intermediate-age stars ($t=$100 Myr-2 Gyr) at larger distances. Older stellar populations dominate in the inner 250 pc.

The Effects of the Overshooting of the Convective Core on Main-sequence Turnoffs of Young- and Intermediate-age Star Clusters

Abstract Recent investigations have shown that the extended main-sequence turnoffs (eMSTOs) are a common feature of intermediate-age star clusters in the Magellanic Clouds. The eMSTOs are also found in the color–magnitude diagram of young-age star clusters. The origin of the eMSTOs is still an open question. Moreover, asteroseismology shows that the value of the overshooting parameter of the convective core is not fixed for the stars with an approximatelly equal mass. Thus the MSTO of star clusters may be affected by the overshooting of the convective core (OVCC). We calculated the effects of the OVCC with different δ ov on the MSTO of young- and intermediate-age star clusters. If δ ov varies between stars in a cluster, the observed eMSTOs of young- and intermediate-age star clusters can be explained well by the effects. The equivalent age spreads of MSTO caused by the OVCC are related to the age of star clusters and are in good agreement with observed results of many clusters. Moreover, the observed eMSTOs of NGC 1856 are reproduced by the coeval populations with different δ ov. The eMSTOs of star clusters may be relevant to the effects of the OVCC. The effects of the OVCC are similar to that of rotation in some respects. However, the effects cannot result in a significant split of the main sequence of young star clusters at . The presence of a rapid rotation can make the split of the main sequence of young star clusters more significant.

The VMC Survey - XXIV. Signatures of tidally stripped stellar populations from the inner Small Magellanic Cloud

We study the luminosity function of intermediate-age red-clump stars using deep, near-infrared photometric data covering $\sim$ 20 deg$^2$ located throughout the central part of the Small Magellanic Cloud (SMC), comprising the main body and the galaxy's eastern wing, based on observations obtained with the VISTA Survey of the Magellanic Clouds (VMC). We identified regions which show a foreground population ($\sim$11.8 $\pm$ 2.0 kpc in front of the main body) in the form of a distance bimodality in the red-clump distribution. The most likely explanation for the origin of this feature is tidal stripping from the SMC rather than the extended stellar haloes of the Magellanic Clouds and/or tidally stripped stars from the Large Magellanic Cloud. The homogeneous and continuous VMC data trace this feature in the direction of the Magellanic Bridge and, particularly, identify (for the first time) the inner region ($\sim$ 2 -- 2.5 kpc from the centre) from where the signatures of interactions start becoming evident. This result provides observational evidence of the formation of the Magellanic Bridge from tidally stripped material from the SMC.

DUSTiNGS. III. DISTRIBUTION OF INTERMEDIATE-AGE AND OLD STELLAR POPULATIONS IN DISKS AND OUTER EXTREMITIES OF DWARF GALAXIES

ABSTRACT We have traced the spatial distributions of intermediate-age and old stars in nine dwarf galaxies in the distant parts of the Local Group, using multi-epoch 3.6 and 4.5 μm data from the DUST in Nearby Galaxies with Spitzer (DUSTiNGS) survey. Using complementary optical imaging from the Hubble Space Telescope, we identify the tip of the red giant branch (TRGB) in the 3.6 μm photometry, separating thermally pulsating asymptotic giant branch stars from the larger red giant branch populations. Unlike the constant TRGB in the I band, at 3.6 μm, the TRGB magnitude varies by ∼0.7 mag, making it unreliable as a distance indicator. The intermediate-age and old stars are well mixed in two-thirds of the sample, with no evidence of a gradient in the ratio of the intermediate-age to old stellar populations outside the central ∼1′–2′. Variable AGB stars are detected in the outer extremities of the galaxies, indicating that chemical enrichment from these dust-producing stars may occur in the outer regions of galaxies with some frequency. Theories of structure formation in dwarf galaxies must account for the lack of radial gradients in intermediate-age populations and the presence of these stars in the outer extremities of dwarfs. Finally, we identify unique features in individual galaxies, such as extended tidal features in Sex A and Sag DIG and a central concentration of AGB stars in the inner regions of NGC 185 and NGC 147.

Star formation history of And XVIII: a dwarf spheroidal galaxy in isolation

We present a photometric study of the Andromeda XVIII dwarf spheroidal galaxy associated with M31, and situated well outside of the virial radius of the M31 halo. The galaxy was resolved into stars with Hubble Space Telescope/Advanced Camera for Surveys revealing the old red giant branch and red clump. With the new observational data we determined the Andromeda XVIII distance to be D = 1.33+-0.08 Mpc using the tip of red giant branch method. Thus, the dwarf is situated at the distance of 579 kpc from M31. We model the star formation history of Andromeda XVIII from the stellar photometry and Padova theoretical stellar isochrones. An ancient burst of star formation occurred 12-14 Gyr ago. There is no sign of recent/ongoing star formation in the last 1.5 Gyr. The mass fractions of the ancient and intermediate age stars are 34 and 66 per cent, respectively, and the total stellar mass is 4.2x10^6 Msun. It is probable that the galaxy has not experienced an interaction with M31 in the past. We also discuss star formation processes of dSphs KKR 25, KKs 03, as well as dTr KK 258. Their star formation histories were uniformly measured by us from HST/ACS observations. All the galaxies are situated well beyond the Local Group and the two dSphs KKR 25 and KKs 03 are extremely isolated. Evidently, the evolution of these objects has proceeded without influence of neighbours.

The tight subgiant branch of the intermediate-age star cluster NGC 411 implies a single-aged stellar population

The presence of extended main-sequence turn-off (eMSTO) regions in intermediate-age star clusters in the Large and Small Magellanic Clouds is often interpreted as resulting from extended star-formation histories (SFHs), lasting $\geq$ 300 Myr. This strongly conflicts with the traditional view of the dominant star-formation mode in stellar clusters, which are thought of as single-aged stellar populations. Here we present a test of this interpretation by exploring the morphology of the subgiant branch (SGB) of NGC 411, which hosts possibly the most extended eMSTO among all known intermediate-age star clusters. We show that the width of the NGC 411 SGB favours the single-aged stellar population interpretation and rules out an extended SFH. In addition, when considering the red clump (RC) morphology and adopting the unproven premise that the widths of all features in the colour--magnitude diagram are determined by an underlying range in ages, we find that the SFH implied is still very close to that resulting from a single-aged stellar population, with a minor fraction of stars scattering to younger ages compared with the bulk of the population. The SFHs derived from the SGB and RC are both inconsistent with the SFH derived from the eMSTO region. NGC 411 has a very low escape velocity and it has unlikely undergone significant mass loss at an early stage, thus indicating that it may lack the capacity to capture most of its initial, expelled gas from stellar evolutionary processes, a condition often required for extended SFHs to take root.

A HUBBLE SPACE TELESCOPE STUDY OF THE ENIGMATIC MILKY WAY HALO GLOBULAR CLUSTER CRATER*

ABSTRACT We analyze the resolved stellar populations of the faint stellar system, Crater, based on deep optical imaging taken with the Advanced Camera for Surveys on the Hubble Space Telescope. Crater’s color–magnitude diagram (CMD) extends ∼4 mag below the oldest main-sequence (MS) turnoff. Structurally, we find that Crater has a half-light radius of ∼20 pc and no evidence for tidal distortions. We model Crater’s CMD as a simple stellar population (SSP) and alternatively by solving for its full star formation history. In both cases, Crater is well described by an SSP with an age of ∼7.5 Gyr, a metallicity of [M/H] ∼ −1.65, a total stellar mass of , and a luminosity of , located at a distance of d ∼ 145 kpc, with modest uncertainties due to differences in the underlying stellar evolution models. We argue that the sparse sampling of stars above the turnoff and subgiant branch are likely to be 1.0–1.4 blue stragglers and their evolved descendants, as opposed to intermediate-age MS stars. We find that Crater is an unusually young cluster given its location in the Galaxy’s outer halo. We discuss scenarios for Crater’s origin, including the possibility of being stripped from the SMC or the accretion from lower-mass dwarfs such as Leo I or Carina. Despite uncertainty over its progenitor system, Crater appears to have been incorporated into the Galaxy more recently than z ∼ 1 (8 Gyr ago), providing an important new constraint on the accretion history of the Galaxy.