Horizontal Branch Morphology Research Articles

CN variations in NGC 7006

Rotationally induced mixing with subsequent dredge-up of nucleosynthesized material is discussed as a second parameter of the horizontal branch morphology in globular clusters. CNO abundances have been proposed as tracers of the dredge up of processed material. \ngc is a prominent example of a second parameter GC: Its HB morphology is too red for its metallicity. We present spectroscopic measurements of CN molecular band strengths S(3839) and CH band CH(4300) strengths for 12 giants in \ngc to test rotationally-driven mixing as a second parameter in this cluster. Our observations reveal (i) a scatter in star-to-star CN absorption strengths with the same amplitude as seen in other GCs of the same metallicity, but different HB morphologies; (ii) a possible continuous distribution of CN absorption strength with a preference for CN-enriched stars, and (iii) a possible weak radial gradient in the number ratio of CN-strong and CN-weak stars. We argue against the hypothesis that CN-variations are directly correlated with the second parameter effect of the HB morphology. However, the small sample of stars measured in \ngc prevents us from drawing firm conclusions. Finally, we identify one star of our sample as a foreground dwarf carbon star.

Blue Straggler Stars: A Direct Comparison of Star Counts and Population Ratios in Six Galactic Globular Clusters

The central regions of six Galactic globular clusters (GGCs) (M3, M80, M10, M13, M92, and NGC 288) have been imaged using HST-WFPC2 and the ultraviolet filters (F255W and F336W). The selected sample covers a large range in both central density (log ρ0) and metallicity ([Fe/H]). In this paper, we present a direct cluster-to-cluster comparison of the blue straggler star (BSS) population as selected from (m255, m255-m336) color magnitude diagrams. We have found (1) that BSSs in three of the clusters (M3, M80, M92) are much more concentrated toward the center of the cluster than the red giants; because of the smaller BSS samples for the other clusters, we can only note that the BSS radial distributions are consistent with central concentration; and (2) that the specific frequency of BSSs varies greatly from cluster to cluster. The most interesting result is that the two clusters with largest BSS specific frequency are at the central density extremes of our sample: NGC 288 (lowest central density) and M80 (highest). This evidence, together with the comparison with theoretical collisional models, suggests that both stellar interactions in high-density cluster cores and at least one other alternate channel operating in low-density GGCs play an important role in the production of BSSs. We also note a possible connection between horizontal-branch morphology and blue straggler luminosity functions in these six clusters.

Variable Stars in Metal-rich Globular Clusters. II. NGC 6316

We present time-series VI photometry of the metal-rich globular cluster NGC 6316. Our color-magnitude diagrams show a predominantly red-clump horizontal branch morphology with no evidence of the blue extensions seen in NGC 6388 and NGC 6441. Our data are in good agreement with published estimates of the reddening and metallicity provided the cluster has an absolute distance modulus of 15.50 mag. We have discovered more than one dozen long-period variable stars in our field of view, and argue that at least seven of them are members of NGC 6316. We have also discovered four RR Lyrae variables, although only one of them probably belongs to the cluster. The specific frequency of RR Lyrae stars in NGC 6316 is found to be less than 0.4, making it clear that this cluster is a normal, metal-rich globular and is not an analog of NGC 6388 and NGC 6441. We recommend long-term photometric monitoring of NGC 6316 to clarify the nature of the RR Lyrae member candidate as well as the pulsation classes of the long-period variables.

Far-Ultraviolet Emission from Elliptical Galaxies at [CLC][ITAL]z[/ITAL][/CLC] = 0.33

We present far-ultraviolet (far-UV) images of the rich galaxy cluster ZwCl1358.1+6245, taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). When combined with archival HST observations, our data provide a measurement of the UV-to-optical flux ratio in 8 early-type galaxies at z=0.33. Because the UV flux originates in a population of evolved, hot, horizontal branch (HB) stars, this ratio is potentially one of the most sensitive tracers of age in old populations -- it is expected to fade rapidly with lookback time. We find that the UV emission in these galaxies, at a lookback time of 3.9 Gyr, is significantly weaker than it is in the current epoch, yet similar to that in galaxies at a lookback time of 5.6 Gyr. Taken at face value, these measurements imply different formation epochs for the massive ellipticals in these clusters, but an alternative explanation is a floor in the UV emission due to a dispersion in the parameters that govern HB morphology.

The Draco and Ursa Minor Dwarf Spheroidal Galaxies: A Comparative Study

We present (V, I) photometry of two wide (≃25 × 25 arcmin2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided [(m-M)0(UMi) = 19.41 ± 0.12 and (m-M)0(Dra) = 19.84 ± 0.14], and a comparative study of their evolved stellar population is presented. We detect for the first time the red giant branch (RGB) bump in the luminosity function of UMi (V = 19.40 ± 0.06), while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies, and an accurate analysis to determine the intrinsic metallicity spread is performed by means of artificial star experiments. The adopted method is insensitive to stars more metal poor than [Fe/H] ∼ -2.5, and it rests on the assumption that the age spread in the considered populations is small (i.e., the impact of the actual age spread on the colors of the RGB stars is negligible). We find that while the average metallicity of the two galaxies is similar (⟨[Fe/H]⟩UMi = -1.8 and ⟨Fe/H]⟩Dra = -1.7), the metallicity distributions are significantly different, having different peak values ([Fe/H] = -1.9 and [Fe/H] = -1.6) and different maximum metallicities. We suggest that such differences may be partly responsible for the difference in horizontal-branch morphology between the two galaxies. The intrinsic metallicity 1 σ spread is σi = 0.10 in UMi and σi = 0.13 in Draco. We demonstrate that the inner region of UMi is significantly structured, at odds with what is expected for a system in dynamical equilibrium. In particular, we show that the main density peak of UMi is off-center with respect to the center of symmetry of the whole galaxy and shows a much lower ellipticity with respect to the rest of the galaxy. Moreover, UMi stars are shown to be clustered according to two different characteristic clustering scales, as opposed to Draco, which instead has a very symmetric and smooth density profile. The possible consequences of this striking structural difference on our ideas about galaxy formation are briefly discussed. Combining our distance modulus with the more recent estimates of the total luminosity of UMi, we find that the mass-to-light ratio (M/L) of this galaxy may be as low as M/L ∼ 7, a factor of 5–10 lower than current estimates.

Horizontal-Branch Morphology and the Photometric Evolution of Old Stellar Populations

Theoretical integrated broadband colors ranging from far-UV to near-IR have been computed for old stellar systems from our evolutionary population synthesis code. These models take into account, for the first time, the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity. Our models show that some temperature-sensitive color indices are significantly affected by the presence of blue HB stars. In particular, B-V does not become monotonically redder as metallicity increases at given ages, but becomes bluer by as much as ~0.15 mag because of the contribution from blue HB stars. Similar trends are also found in the Washington photometric system. In addition to appropriate age-sensitive spectrophotometric indices, the use of far-UV to optical colors is proposed as a powerful age diagnostic for old stellar systems with differing HB morphologies. Our models are calibrated in the B-V, V-I, C-T1, and M-T1 versus [Fe/H] planes, using low-reddened Galactic globular clusters (GCs) [E(B-V) < 0.2], and the relative age difference between the older inner halo Galactic GCs and younger outer halo counterparts is well reproduced. Several empirical linear color-metallicity transformation relations are assessed with our models, and it is noted that they may not be safely used to estimate metallicity if there are sizable age differences among GCs within and between galaxies. M31 GCs are found to be fundamentally similar to those in the Milky Way, not only in the optical to near-IR range, but also in the UV range. For globular cluster systems in two nearby giant ellipticals, M87 and NGC 1399, the current available photometric data in the literature do not appear sufficient to provide robust age discrimination. It is anticipated, however, that the detailed population models presented here, coupled with further precise spectrophotometric observations of globular cluster systems in external galaxies from the large ground-based telescopes and space UV facilities, will enable us to accurately estimate their ages and metallicities.

Integrated spectroscopy of bulge globular clusters and fields

We present a comprehensive spectroscopic study of the integrated light of metal-rich Galactic globular clusters and the stellar population in the Galactic bulge. We measure line indices which are defined by the Lick standard system and compare index strengths of the clusters and Galactic bulge. Both metal-rich globular clusters and the bulge are similar in most of the indices, except for the CN index. We find a significant enhancement in the CNFe index ratio in metal-rich globular clusters compared with the Galactic bulge. The mean iron index Fe of the two metal-rich globular clusters NGC 6528 and NGC 6553 is comparable with the mean iron index of the bulge. Index ratios such as MgbFe, MgFe, Ca 4227Fe, and TiOFe, are comparable in both stellar population indicating similar enhancements in individual elements which are traced by the indices. From the globular cluster data we fully empirically calibrate several metallicity-sensitive indices as a function of [Fe/H] and find tightest correlations for the Mg2 index and the composite [MgFe] index. We find that all indices show a similar behavior with galactocentric radius, except for the Balmer series, which show a large scatter at all radii. However, the scatter is entirely consistent with the cluster-to-cluster variations in the horizontal branch morphology.

Testing stellar population models with star clusters in the Large Magellanic Cloud

We present high signal-to-noise integrated spectra of 24 star clusters in the Large Magellanic Cloud (LMC), obtained using the FLAIR spectrograph at the UK Schmidt telescope. The spectra have been placed onto the Lick/IDS system in order to test the calibration of Simple Stellar Population (SSP) models (Maraston & Thomas 2000; Kurth, Fritz-von Alvensleben & Fricke 1999). We have compared the SSP-predicted metallicities of the clusters with those from the literature, predominantly taken from the Ca-Triplet spectroscopy of Olszewski et al.(1991). We find that there is good agreement between the metallicities in the range - 2.10 ≤ (Fe/H) ≤ 0. However, the Mg2 index (and to a lesser degree Mg b) systematically predict higher metallicities (up to +0.5 dex higher) than h Fei . Among the possible explanations for this are that the LMC clusters possess (�/Fe) > 0. Metallicities are presented for eleven LMC clusters which have no previous measurements. We compare SSP ages for the clusters, derived from the H�, H and HLick/IDS indices, with the available literature data, and find good agreement for the vast ma- jority. This includes six old globular clusters in our sample, which have ages consistent with their HST CMD ages and/or integrated colours. However, two globular clusters, NGC 1754 and NGC 2005, identified as old (∼ 15 Gyr) on the basis of HST CMDs, have Hline-strengths which lead ages which are too young (∼ 8 and ∼ 6 Gyr re- spectively). These findings are inconsistent with their CMD-derived values at the 3� level. Comparison between the horizontal branch morphology and the Balmer line- strengths of these clusters suggests that the presence of blue horizontal branch stars has increased their Balmer indices by up to ∼ 1.0 u We conclude that the Lick/IDS indices, used in conjunction with contemporary SSP models, are able to reproduce the ages and metallicities of the LMC clusters reassuringly well. The required extrapolations of the fitting-functions and stellar li- braries in the models to younger ages and low metallicities do not lead to serious systematic errors. However, due to the significant contribution of horizontal branch stars to Balmer indices, SSP model ages derived for metal-poor globular clusters are ambiguous without a priori knowledge of horizontal branch morphology.

The Dwarf Spheroidal Companions to M31: WFPC2 Observations of Andromeda III

The Hubble Space Telescope WFPC2 camera has been used to image Andromeda?II, a dwarf spheroidal (dSph) companion to M31. The resulting color-magnitude (c-m) diagrams reveal the morphology of the horizontal branch (HB) in this dwarf galaxy. We find that like Andromeda?I, and like most of the Galactic dSph companions, the HB morphology of And?II is predominantly red. Unlike And?I, however, there is no evidence for a radial gradient in HB morphology in the And?II data. Based on a comparison with a combination of standard Galactic globular cluster c-m diagrams scaled to reproduce the And?II mean abundance and abundance dispersion, we interpret the observed HB morphology of And?II as indicating that at least 50% of the total stellar population is younger than the age of the globular clusters. This inference is strengthened by the small number of confirmed upper-AGB carbon stars in And?II. The relatively faint luminosities (Mbol ? -4.1) of these stars, however, suggest an age or ages nearer 6?9 Gyr, rather than 1?3 Gyr, for this population. On the other hand, the existence of blue HB and RR?Lyrae variable stars in And?II argues for the existence of an additional old (age > 10 Gyr) population in this dSph. Thus, And?II has had an extended epoch of star formation like many of the Galactic dSphs. The mean magnitude of the blue HB in And?II suggests (m-M)0 = 24.17 ? 0.06 and that And?II is 125 ? 60 kpc closer than M31 along the line of sight. This confirms the association of And?II with M31, rather than with M33 to which And?II lies closer on the sky. The true distance of And?II from the center of M31 is between ~160 and ~230 kpc, comparable to the Galactocentric distances of Fornax and of the Leo dSphs. With the current samples of dSph companions, the size of the Galaxy's and M31's dSph satellite systems are comparable, with outer radii of order 250 kpc. The And?II red giant branch colors yield a mean abundance of [Fe/H] = -1.49 ? 0.11 and a surprisingly large internal abundance spread, which can be characterised by ?int([Fe/H]) ? 0.36 dex. Both these values are in good agreement with the recent ground-based spectroscopic study of C?t? et al. The And?II abundance dispersion found here is considerably larger than that derived for And?I from an identical analysis of similar data (?int([Fe/H]) = 0.21 dex). Thus, despite having very similar luminosities and mean metal abundances, these two M31 dSph companions have clearly had different chemical evolution histories. We find that we cannot model the abundance distribution in And?II with single component simple chemical enrichment models. However, we can reproduce the form of the distribution if we assume two components, each with a simple model abundance distribution. The metal-poor component has mean abundance log(z/zsun) = -1.6, while the metal-rich one has mean abundance log(z/zsun) = -0.95 and is outnumbered by the metal-poor population by a ratio of ~2.3 to 1. We end by concluding that the diversity of evolutionary histories evident among the Galactic dSph companions is now also firmly established among the dSph satellites of M31. An Appendix discusses minor revisions to our earlier And?I results that arise from the calibration and analysis techniques adopted in this paper. In particular, our comparisons with ground-based photometry indicate that the zero point for the WFPC2 F450W to B transformation should be modified, by 0.055 mag, to produce fainter B magnitudes and thus redder B-V colors.

The Stellar Populations of the Cetus Dwarf Spheroidal Galaxy

We present Hubble Space Telescope Wide Field Planetary Camera 2 photometry in the V and I passbands of the recently discovered Local Group dwarf spheroidal galaxy in Cetus. Our color-magnitude diagram extends from above the first-ascent red giant branch (RGB) tip to approximately half a magnitude below the horizontal branch (HB). Given a reddening of E(B-V) = 0.03, the magnitude of the RGB tip yields a distance modulus of (m-M)0 = 24.46 ± 0.14. After applying the reddening and distance modulus, we have utilized the color distribution of RGB stars to determine a mean metal abundance of [Fe/H] = -1.7 on the Zinn & West scale, with an intrinsic internal abundance dispersion of ~0.2 dex. An indirect calculation of the HB morphology of Cetus based on the mean dereddened HB color yields (B-R)/(B+V+R) = -0.91 ± 0.09, which represents an HB that is redder than what can be attributed solely to Cetus's metal abundance. Thus, Cetus is affected by the in which another parameter besides metallicity is controlling the HB morphology. If we adopt the conventional age hypothesis explanation for the second-parameter effect, then this implies that Cetus is 2-3 Gyr younger than Galactic globular clusters at its metallicity.

Relative Ages from Horizontal-Branch Morphology: Revisited

We present our recent revision of model constructions for the horizontal-branch (HB) morphology of globular clusters, which suggests the HB morphology is more sensitive to age compared to our earlier models. We also present our high precision CCD photometry for the classic second parameter pair M3 and M13. The relative age dating based on this photometry indicates that M13 is indeed older than M3 by 1.7 Gyr. This is consistent with the age difference predicted from our new models, which provides a further support that the HB morphology is a reliable age indicator in most population II stellar systems.

Second Parameter Effects in and between M3 and Palomar 3

We study the globular clusters M3 and Palomar 3 as a “second parameter (2ndP) pair,” showing that: i) M3 has a surprisingly strong internal 2ndP; ii) The dispersion in mass on the Pal 3 horizontal branch (HB) is intrinsically very small, leading to the most apparent differences in HB morphology between M3 and Pal 3; iii) Ignoring the difference in HB mass dispersion between M3 and Pal 3, their relative HB types can be accounted for by a fairly small difference in age, of order 0.5–1 Gyr.

The Hβ Index As a Chronometer of Extragalactic Globular Cluster Systems

For the first time, we have taken into account the detailed systematic variation of horizontal-branch (HB) morphology with age and metallicity in our population synthesis models and they result that the integrated Hβ index is significantly affected by the presence of blue HB stars. As a matter of fact, due to the systematic HB morphology variation, it is found that Hβ does not monotonically decrease as metallicity increases at given ages, but shows a kind of wavy feature. According to our models, a systematic difference between the globular cluster system in the Milky Way Galaxy and that in NGC 1399 in the Hβ vs. Mg2 plane is explained if globular cluster systems in giant elliptical galaxies are a couple of billion years older, in the mean, than the Galactic counterpart.

RR Lyrae Variables in Local Group Galaxies: M31

AbstractCandidate RR Lyrae variables have been identified for the first time in four globular clusters of the Andromeda galaxy (M31). Using HST-WFPC2 archive observations we have found 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322 (Sargent et al., 1977), respectively. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters.

A Survey of M31 Globular Clusters Using WFPC2 on board HST

We report new, as yet unpublished HST/WFPC2 V, I photometry of 9 globular clusters in M31. These are part of a total sample of 19 clusters on M31 with well observed CMDs from HST. The clusters have the full range of horizontal branch morphologies from blue HB to red clump, but none are found with an extreme (blue only) horizontal branch. Plots of HB type vs [Fe/H] are similar to those of Galactic clusters, including a hint of second parameter clusters. Sixteen halo fields adjacent to the observed clusters included in our images have been analysed. The M31 halo metallicity distribution peaks at [Fe/H]=-0.7 dex with a tail toward low metallicity, resembling the halo of NGC 5128; metal-rich giants appear to be centrally concentrated.

The Formation Chronology of Local Group Spiral Galaxies

The ‘Second Parameter Effect’ (2ndPE) has long been recognized as an important probe into the formation of spiral galaxies. The concept that the horizontal branch morphologies of globular clusters are primarily affected by metal abundance in the inner halo (RGC&lt;8 kpc) of the Galaxy but require an additional parameter (probably cluster age) to explain their behavior in the outer halo (RGC &gt; 8 kpc), suggests that the former experienced a rapid monotonic collapse while the latter underwent a slower chaotic formation scenario. As such, in the Milky Way, the so-called second parameter boundary is located at 8 kpc. We find that, in the other Local Group spirals — M31 and M33 — this boundary lies at ∼40 kpc and ∼0 kpc, respectively. We therefore speculate that the boundary delimiting rapid monotonic halo collapse from the chaotic accretion of dwarf galaxy fragments is inversely related to the mass of the spiral galaxy.

CCD Photometry of the Classic Second-Parameter Globular Clusters M3 and M13

We present high-precision V, B-V color-magnitude diagrams (CMDs) for the classic second parameter globular clusters M3 and M13 from wide-field deep CCD photometry. The data for the two clusters were obtained during the same photometric nights with the same instrument, allowing us to determine accurate relative ages. Based on a differential comparison of the CMDs using the Delta (B-V) method, an age difference of 1.7 +/- 0.7 Gyr is obtained between these two clusters. We compare this result with our updated horizontal-branch (HB) population models, which confirm that the observed age difference can produce the difference in HB morphology between the clusters. This provides further evidence that age is the dominant second parameter that influences HB morphology.

The Star Formation History of the Large Magellanic Cloud

Using WFPC2 aboard the Hubble Space Telescope, we have created deep color-magnitude diagrams in the V and I passbands for approximately 100,000 stars in a field at the center of the LMC bar and another in the disk. The main--sequence luminosity functions (LFs) from 19 mag < V < 23.5 mag, the red clump and horizontal branch morphologies, and the differential Hess diagram of the two fields all strongly imply that the disk and bar have significantly different star-formation histories (SFHs). The disk's SFH has been relatively smooth and continuous over the last 15 Gyr while the bar's SFH was dominated by star formation episodes at intermediate ages. Comparison of the LF against predictions based on Padova theoretical stellar evolution models and an assumed age-metallicity relationship allows us to identify the dominant stellar populations in the bar with episodes of star formation that occurred from 4 to 6 and 1 to 2 Gyr ago. These events accounted, respectively, for approximately 25% and 15% of its stellar mass. The disk field may share a mild enhancement in SF for the younger episode, and thus we identify the 4 to 6 Gyr episode with the formation of the LMC bar.

Age as the Second Parameter in NGC 288/NGC 362? II. The Horizontal Branch Revisited

We revisit the "second-parameter" pair of globular clusters NGC 288/362 on the basis of theoretical models for red giant branch (RGB) and horizontal branch (HB) stars. The results of the most extensive set of RGB/HB simulations computed so far for these clusters are presented for two different metallicities. Using several different analytical mass-loss formulae for RGB stars, we derive relative "HB morphology ages." We compare them with the relative main-sequence turnoff ages derived by application of the "bridge test" in Paper I, where it was found that NGC 288 is 2 ± 1 Gyr older than NGC 362. We find that adoption of a higher metallicity ([Fe/H] ≈ -1.2), as favored by the Carretta & Gratton metallicity scale, makes age a much more plausible second-parameter candidate for this pair than is the case when a lower metallicity ([Fe/H] ≈ -1.5), closer to the Zinn & West scale, is adopted. However, while the different HB morphology of these two clusters can be reproduced by canonical HB models with [Fe/H] ≈ -1.2 and an age difference of 2 Gyr, this explanation is not without difficulty. In particular, we confirm previous suggestions that canonical models are unable to reproduce the detailed HB morphology of NGC 288 at its red end, for as yet unknown reasons. Finally, we show that the mass dispersion on the HB of NGC 362 is substantially larger than for NGC 288 and suggest that there is a correlation between the mass dispersion on the HB phase and the central density of globular clusters. This is presumably related to the way environmental effects affect RGB mass loss—another major second-parameter candidate. We argue that, if confirmed, this central density–HB mass dispersion correlation will have to be taken into account in order to conclusively determine whether age may be considered the (sole) second parameter of HB morphology for this (and other) second-parameter pair(s).

RR Lyrae Variables in the Globular Clusters of M31: A First Detection of Likely Candidates

The purpose of this paper is to show that RR Lyrae variables exist and can be detected in M31 globular clusters. We report on the first tentative identification of RR Lyrae candidates in four globular clusters of the Andromeda galaxy, i.e. G11, G33, G64 and G322. Based on HST-WFPC2 archive observations in the F555W and F814W filters spanning a total interval of about 5 consecutive hours we find evidence for 2, 4, 11 and 8 RR Lyrae variables of both ab and c Bailey types in G11, G33, G64 and G322, respectively. Several more candidates can be found by relaxing slightly the selection criteria. These numbers are quite consistent with the horizontal branch morphology exhibited by the four clusters, starting from the very blue HB in G11, and progressively moving to redder HBs in G64, G33 and G322.