Carme Gallart

Chemical abundance analysis of the open clusters Cr 110, NGC 2099 (M 37), NGC 2420, NGC 7789, and M 67 (NGC 2682)

Context. The present number of Galactic open clusters that have high resolution abundance determinations, not only of [Fe/H], but also of other key elements, is largely insufficient to enable a clear modeling of the Galactic disk chemical evolution. Aims. To increase the number of Galactic open clusters with high quality measurements. Methods. We obtained high resolution (R ∼ 30 000), high quality (S /N ∼ 50–100 per pixel), echelle spectra with the fiber spectrograph FOCES, at Calar Alto, Spain, for three red clump stars in each of five Open Clusters. We used the classical equivalent width analysis method to obtain accurate abundances of sixteen elements: Al, Ba, Ca, Co, Cr, Fe, La, Mg, Na, Nd, Ni, Sc, Si, Ti, V, and Y. We also derived the oxygen abundance using spectral synthesis of the 6300 A forbidden line. Results. Three of the clusters were never studied previously with high resolution spectroscopy: we found [Fe/H] =+ 0.03 ± 0.02 (±0.10) dex for Cr 110; [Fe/H] =+ 0.01 ± 0.05 (±0.10) dex for NGC 2099 (M 37), and [Fe/H] = −0.05 ± 0.03 (±0.10) dex for NGC 2420. This last finding is higher than typical literature estimates by 0.2–0.3 dex approximately and in closer agreement with Galactic trends. For the remaining clusters, we find that [Fe/H] =+ 0.05 ± 0.02 (±0.10) dex for M 67 and [Fe/H] =+ 0.04 ± 0.07 (±0.10) dex for NGC 7789. Accurate (to ∼0.5 km s −1 ) radial velocities were measured for all targets, and we provide the first velocity estimate derived from high resolution data for Cr 110, � Vr� = 41.0 ± 3. 8k m s −1 . Conclusions. With our analysis of the new clusters Cr 110, NGC 2099, and NGC 2420, we increase the sample of clusters with highresolution-based abundances by 5%. All our program stars show abundance patterns which are typical of open clusters, very close to solar with few exceptions. This is true for all the iron-peak and s-process elements considered, and no significant α-enhancement is found. No significant (anti-)correlations for Na, Al, Mg, and O abundances are found. If anticorrelations are present, the involved spreads must be <0.2 dex. We then compile high resolution data of 57 OC from the literature and find a gradient of [Fe/H] with Galactocentric radius of –0.06 ± 0.02 dex kpc −1 , in agreement with past work and with results for Cepheids and B stars in the same range. A change of slope is seen outside RGC = 12 kpc and [α/Fe] shows a tendency to increase with RGC. We also confirm the absence of a significant age-metallicity relation, finding slopes of –2.6 ± 1.1 × 10 −11 dex Gyr −1 and 1.1 ± 5.0 × 10 −11 dex Gyr −1 for [Fe/H] and [α/Fe] respectively.

Leo A: A Late-blooming Survivor of the Epoch of Reionization in the Local Group

As part of a major program to use isolated Local Group dwarf galaxies as near-field probes of cosmology, we have obtained deep images of the dwarf irregular galaxy Leo A with the Advanced Camera for Surveys aboard the Hubble Space Telescope. From these images we have constructed a color-magnitude diagram (CMD) reaching apparent [ absolute] magnitudes of, the deepest ever achieved for any (M-475, M-814) greater than or similar to (29.0 [+ 4.4], 27.9 [+ 3.4]) irregular galaxy beyond the Magellanic Clouds. We derive the star formation rate ( SFR) as a function of time over the entire history of the galaxy. We find that over 90% of all the star formation that ever occurred in Leo A happened more recently than 8 Gyr ago. The CMD shows only a very small amount of star formation in the first few billion years after the big bang; a possible burst at the oldest ages cannot be claimed with high confidence. The peak SFR occurred approximate to 1.5-4 Gyr ago, at a level 5-10 times the current value. Our modeling indicates that Leo A has experienced very little metallicity evolution; the mean inferred metallicity is consistent with measurements of the present-day gas-phase oxygen abundance. We cannot exclude a scenario in which all of the ancient star formation occurred prior to the end of the era of reionization, but it seems unlikely that the lack of star formation prior to approximate to 8 Gyr ago was due to early loss or exhaustion of the in situ gas reservoir.

IAC-STAR: A Code for Synthetic Color-Magnitude Diagram Computation

The code IAC-STAR is presented. It generates synthetic H-R and color-magnitude diagrams (CMDs) and is mainly aimed at star formation history studies in nearby galaxies. Composite stellar populations are calculated on a star-by-star basis, by computing the luminosity, effective temperature, and gravity of each star by direct bilogarithmic interpolation in the metallicity and age grid of a library of stellar evolution tracks. Visual (broadband and Hubble Space Telescope) and infrared magnitudes are also provided for each star after applying bolometric corrections. The Padua stellar evolution libraries of Bertelli et al. and Girardi et al., respectively, and the Teramo stellar evolution library by Pietrinferni et al., as well as various bolometric corrections libraries are used in the current version. A variety of star formation rate functions, initial mass functions, and chemical enrichment laws are allowed, and binary stars can be computed. Although the main motivation of the code is the computation of synthetic CMDs, it also provides integrated masses, luminosities, and magnitudes, as well as surface brightness fluctuation luminosities and magnitudes for the total synthetic stellar population, and therefore it can also be used for population synthesis research. The code is offered for free use and can be accessed at the IAC-STAR Web site. The only requirement is that this paper be referenced and credited as indicated there.

The chemical enrichment history of the large magellanic cloud

Ca II triplet spectroscopy has been used to derive stellar metallicities for individual stars in four Large Magellanic Cloud fields situated at galactocentric distances of 3°, 5°, 6°, and 8° to the north of the bar. Observed metallicity distributions show a well-defined peak, with a tail toward low metallicities. The mean metallicity remains constant until 6° ([Fe/H] ~ −0.5 dex), while for the outermost field, at 8°, the mean metallicity is substantially lower than in the rest of the disk ([Fe/H] ~ −0.8 dex). The combination of spectroscopy with deep CCD photometry has allowed us to break the RGB age-metallicity degeneracy and compute the ages for the objects observed spectroscopically. The obtained age-metallicity relationships (AMRs) for our four fields are statistically indistinguishable. We conclude that the lower mean metallicity in the outermost field is a consequence of it having a lower fraction of intermediate-age stars, which are more metal-rich than the older stars. The disk AMR is similar to that for clusters. However, the lack of objects with ages between 3 and 10 Gyr is not observed in the field population. Finally, we used data from the literature to derive consistently the AMR of the bar. Simple chemical evolution models have been used to reproduce the observed AMRs with the purpose of investigating which mechanism has participated in the evolution of the disk and bar. We find that while the disk AMR is well reproduced by close-box models or models with a small degree of outflow, that of the bar is only reproduced by models with combination of infall and outflow.

THE ACS LCID PROJECT. V. THE STAR FORMATION HISTORY OF THE DWARF GALAXY LGS-3: CLUES TO COSMIC REIONIZATION AND FEEDBACK*

We present an analysis of the star formation history (SFH) of the transition-type (dIrr/dSph) Local Group galaxy LGS-3 (Pisces) based on deep photometry obtained with the Advanced Camera for Surveys onboard the Hubble Space Telescope. Our observations reach the oldest main-sequence turnoffs at high signal to noise, allowing a time resolution at the oldest ages of σ ~ 1.1 Gyr. Our analysis, based on three different SFH codes, shows that the SFH of LGS-3 is dominated by a main episode ~11.7 Gyr ago with a duration of ~1.4 Gyr. Subsequently, LGS-3 continued forming stars until the present, although at a much lower rate. Roughly 90% of the stars in LGS-3 were formed in the initial episode of star formation. Extensive tests of self-consistency, uniqueness, and stability of the solution have been performed together with the IAC-star/IAC-pop/MinnIAC codes, and these results are found to be independent of the photometric reduction package, the stellar evolution library, and the SFH recovery method. There is little evidence of chemical enrichment during the initial episode of star formation, after which the metallicity increased more steeply reaching a present-day value of Z ~ 0.0025. This suggests a scenario in which LGS-3 first formed stars mainly from infalling fresh gas, and after about 9 Gyr ago, from a larger fraction of recycled gas. The lack of early chemical enrichment is in contrast to that observed in the isolated dSph galaxies of comparable luminosity, implying that the dSphs were more massive and subjected to more tidal stripping. We compare the SFH of LGS-3 with expectations from cosmological models. Most or all the star formation was produced in LGS-3 after the reionization epoch, assumed to be completed at z ~ 6 or ~12.7 Gyr ago. The total mass of the galaxy is estimated to be between 2 and 4 × 108 M ☉ corresponding to circular velocities between 28 km s–1 and 36 km s–1. These values are close to but somewhat above the limit of 30 km s–1 below which the UV background is expected to prevent any star formation after reionization. Feedback from supernovae (SNe) associated with the initial episode of star formation (mechanical luminosity from SNe Lw = 5.3 × 1038 erg s–1) is probably inadequate to completely blow away the gas. However, the combined effects of SN feedback and UV background heating might be expected to completely halt star formation at the reionization epoch for the low mass of LGS-3; this suggests that self-shielding is important to the early evolution of galaxies in this mass range.

The ACS LCID Project: On the Origin of Dwarf Galaxy Types—A Manifestation of the Halo Assembly Bias?

We discuss how knowledge of the whole evolutionary history of dwarf galaxies, including details on the early star formation events, can provide insight on the origin of the different dwarf galaxy types. We suggest that these types may be imprinted by the early conditions of formation rather than only being the result of a recent morphological transformation driven by environmental effects. We present precise star formation histories of a sample of Local Group dwarf galaxies, derived from color-magnitude diagrams reaching the oldest main-sequence turnoffs. We argue that these galaxies can be assigned to two basic types: fast dwarfs that started their evolution with a dominant and short star formation event and slow dwarfs that formed a small fraction of their stars early and have continued forming stars until the present time (or almost). These two different evolutionary paths do not map directly onto the present-day morphology (dwarf spheroidal versus dwarf irregular). Slow and fast dwarfs also differ in their inferred past location relative to the Milky Way and/or M31, which hints that slow dwarfs were generally assembled in lower-density environments than fast dwarfs. We propose that the distinction between a fast and slow dwarf galaxy primarily reflects the characteristic density of the environment where they form. At a later stage, interaction with a large host galaxy may play a role in the final gas removal and ultimate termination of star formation. Based on observations made with the NASA/ESA HST, which is operated by the AURA, under NASA contract NAS5-26555. Observations associated with programs #8706, #10505, and #10590.

THE CHEMICAL ENRICHMENT HISTORY OF THE SMALL MAGELLANIC CLOUD AND ITS GRADIENTS

We present stellar metallicities derived from Ca II triplet spectroscopy in over 350 red giant branch stars in 13 fields distributed in different positions in the Small Magellanic Cloud, ranging from ~1° to ~4° from its center. In the innermost fields, the average metallicity is [Fe/H] ~–1. This value decreases when we move away toward outermost regions. This is the first detection of a spectroscopic metallicity gradient in this galaxy. We show that the metallicity gradient is related to an age gradient, in the sense that more metal-rich stars, which are also younger, are concentrated in the central regions of the galaxy.

THE ACS LCID PROJECT. VI. THE STAR FORMATION HISTORY OF THE TUCANA dSph AND THE RELATIVE AGES OF THE ISOLATED dSph GALAXIES

We present a detailed study of the star formation history (SFH) of the Tucana dwarf spheroidal galaxy. High-quality, deep HST/ACS data, collected in the framework of the LCID project, allowed us to obtain the deepest color-magnitude diagram to date, reaching the old main-sequence turnoff (F814 ~ 29) with good photometric accuracy. Our analysis, based on three different SFH codes, shows that Tucana is an old and metal-poor stellar system, which experienced a strong initial burst of star formation at a very early epoch (13 Gyr ago) which lasted a maximum of 1 Gyr (sigma value). We are not able to unambiguously answer the question of whether most star formation in Tucana occurred before or after the end of the reionization era, and we analyze alternative scenarios that may explain the transformation of Tucana from a gas-rich galaxy into a dSph. Current measurements of its radial velocity do not preclude that Tucana may have crossed the inner regions of the Local Group (LG) once, and so gas stripping by ram pressure and tides due to a close interaction cannot be ruled out. A single pericenter passage would generate insufficient tidal heating to turn an originally disky dwarf into a true dSph; however, this possibility would be consistent with the observed residual rotation in Tucana. On the other hand, the high star formation rate measured at early times may have injected enough energy into the interstellar medium to blow out a significant fraction of the initial gas content. Gas that is heated but not blown out would also be more easily stripped via ram pressure. We compare the SFH inferred for Tucana with that of Cetus, the other isolated LG dSph galaxy in the LCID sample. We show that the formation time of the bulk of star formation in Cetus is clearly delayed with respect to that of Tucana. This reinforces the conclusion of Monelli et al. that Cetus formed the vast majority of its stars after the end of the reionization era implying, therefore, that small dwarf galaxies are not necessarily strongly affected by reionization, in agreement with many state-of-the-art cosmological models.

ON THE EXTENDED STRUCTURE OF THE PHOENIX DWARF GALAXY

The surface brightness profile in the V band of the Phoenix dwarf galaxy shows two stellar components: an inner one, which contains all the young stars of the galaxy, and an outer one predominantly populated by red stars.Deep color-magnitude diagrams (CMDs), based on Hubble Space Telescope (HST) observations and reaching the oldest turn-offs, are used to analyze the inner and outer stellar components. Results show that, together with an old stellar population, the outer field contains also an intermediate-age population. These results are compatible with a scenario in which star forming regions are shrinking with time (the shrinking scenario). It seems more difficult to support a halo-disk scenario, which would require extended structures populated only by really old stars.

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

The star formation history (SFH) and the properties of the dwarf galaxy LGS3 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 LGS3 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 LGS3 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)x10^(-10) Mo yr^(-1) pc^(-2) or (0.47+/-0.07)x10^(-10) Mo yr^(-1) pc^(-2), depending of the distance, and its average for the entire life of the galaxy, =(1.4+/-0.1)x10^(-10) Mo yr^(-1) pc^(-2). At the present psi(t), the probability of LGS3 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