[Show abstract][Hide abstract] ABSTRACT: NGC288 is a globular cluster with a well-developed blue horizontal branch
covering the u-jump that indicates the onset of diffusion. It is therefore well
suited to study the effects of diffusion in blue horizontal branch (HB) stars.
We compare observed abundances with predictions from stellar evolution models
calculated with diffusion and from stratified atmospheric models. We verify the
effect of using stratified model spectra to derive atmospheric parameters. In
addition, we investigate the nature of the overluminous blue HB stars around
the u-jump. We defined a new photometric index sz from uvby measurements that
is gravity-sensitive between 8000K and 12000K. Using medium-resolution spectra
and Stroemgren photometry, we determined atmospheric parameters (T_eff, log g)
and abundances for the blue HB stars. We used both homogeneous and stratified
model spectra for our spectroscopic analyses. The atmospheric parameters and
masses of the hot HB stars in NGC288 show a behaviour seen also in other
clusters for temperatures between 9000K and 14000K. Outside this temperature
range, however, they instead follow the results found for such stars in omega
Cen. The abundances derived from our observations are for most elements (except
He and P) within the abundance range expected from evolutionary models that
include the effects of atomic diffusion and assume a surface mixed mass of
10^-7 M0. The abundances predicted by stratified model atmospheres are
generally significantly more extreme than observed, except for Mg. When
effective temperatures, surface gravities, and masses are determined with
stratified model spectra, the hotter stars agree better with canonical
evolutionary predictions. Our results show definite promise towards solving the
long-standing problem of surface gravity and mass discrepancies for hot HB
stars, but much work is still needed to arrive at a self-consistent solution.
Astronomy and Astrophysics 03/2014; 565. · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present the results of a Hubble Space Telescope program to search for
pulsating hot subdwarfs in the core of NGC 2808. These observations were
motivated by the recent discovery of such stars in the outskirts of omega Cen.
Both NGC 2808 and omega Cen are massive globular clusters exhibiting complex
stellar populations and large numbers of extreme horizontal branch stars. Our
far-UV photometric monitoring of over 100 hot evolved stars has revealed six
pulsating subdwarfs with periods ranging from 85 to 149 s and UV amplitudes of
2.0 to 6.8%. In the UV color-magnitude diagram of NGC 2808, all six of these
stars lie immediately below the canonical horizontal branch, a region populated
by the subluminous "blue-hook" stars. For three of these six pulsators, we also
have low-resolution far-UV spectroscopy that is sufficient to broadly constrain
their atmospheric abundances and effective temperatures. Curiously, and in
contrast to the omega Cen pulsators, the NGC 2808 pulsators do not exhibit the
spectroscopic or photometric uniformity one might expect from a well-defined
instability strip, although they all fall within a narrow band (0.2 mag) of
The Astrophysical Journal 10/2013; 777(2). · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We are coming to believe that stellar populations in globular clusters are
not as simple as they were once thought to be. A growing amount of photometric
and spectroscopic evidence shows that globular clusters host at least two
different stellar populations. In our contribution to these proceedings we
present the first results of a survey we are conducting to look for the
presence of multiple populations in a significant number of Galactic globular
clusters, using the Str\"omgren system. We intend to photometrically separate
these populations and characterize their radial distributions and extensions.
Memorie della Societa Astronomica Italiana. 01/2013;
[Show abstract][Hide abstract] ABSTRACT: An increasing amount of spectroscopic and photometric evidence is showing
that the stellar populations of globular clusters are not as simple as they
have been considered for many years. The presence of at least two different
populations of stars is being discovered in a growing number of globular
clusters, both in our Galaxy and in others. We have started a series of
observations of Galactic globular clusters using the Str\"omgren photometric
system in order to find the signatures of these multiple populations and
establish their presence in a more complete sample of globular clusters in the
Milky Way, and to study their radial distributions and extensions. We present
here the first results of our survey.
[Show abstract][Hide abstract] ABSTRACT: Recently, the study of globular cluster (GC) CMDs has shown that some of them
harbor multiple populations with different chemical compositions and/or ages.
In the first case, the most common candidate is a spread in the initial helium
abundance, but this quantity is difficult to determine spectroscopically due to
the fact that helium absorption lines are not present in cooler stars, whereas
for hotter GC stars gravitational settling of helium becomes important. As a
consequence, indirect methods to determine the initial Y among populations are
necessary. For that reason, in this series of papers, we investigate the
effects of a Y enrichment in populations covering the range of GC
metallicities. In this first paper, we present the theoretical evolutionary
tracks, isochrones, and ZAHB loci calculated with the Princeton-Goddard-PUC
(PGPUC) stellar evolutionary code, which has been updated with the most recent
input physics and compared with other theoretical databases. The chemical
composition grid covers 9 Z ranging from Z=1.60x10^-4 to 1.57x10^-2, 7 Y from
Y=0.230 to 0.370, and an alpha-element enhancement of [alpha/Fe]=0.3. The
effects of different helium abundances that can be observed in isochrones are:
splits in the MS, differences in the L and Teff of the turn off point, splits
in the SGB being more prominent for lower ages or higher metallicities, splits
in the lower red giant branch being more prominent for higher ages or higher
metallicities, differences in L of the RGB bump (with small changes in Teff),
and differences in L at the RGB tip. At the ZAHB, when Y is increased there is
an increase of L for low Teff, which is affected in different degrees depending
on the age of the GC being studied. Finally, the ZAHB morphology distribution
depending on the age explains how for higher GC metallicities a population with
higher helium abundance could be hidden at the red ZAHB locus.
Astronomy and Astrophysics 08/2012; · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: [Abridged] We present new HST FUV spectroscopy of 24 hot evolved stars in
NGC2808, a massive globular cluster with a large population of "blue-hook"
(BHk) stars. The BHk stars are found in UV color-magnitude diagrams of the most
massive globular clusters, where they fall at luminosities immediately below
the hot end of the horizontal branch (HB), in a region of the HR diagram
unexplained by canonical stellar evolution theory. Using new evolutionary and
atmospheric models, we have shown that these subluminous HB stars are very
likely the progeny of stars that undergo extensive internal mixing during a
late He-core flash on the white dwarf cooling curve. This flash mixing leads to
hotter temperatures and an enormous enhancement of the surface He and C; these
hotter temperatures, together with the decrease in H opacity shortward of the
Lyman limit, make the BHk stars brighter in the EUV while appearing subluminous
in the UV and optical. Our FUV spectroscopy demonstrates that, relative to
normal HB stars at the same color, the BHk stars of NGC2808 are hotter and
greatly enhanced in He and C, thus providing unambiguous evidence of flash
mixing. Although the C abundance in the BHk stars is orders of magnitude larger
than that in the normal HB stars, the C abundance in both the BHk and normal HB
stars appears to be affected by gravitational settling. The variations seen in
Si and the Fe-peak elements also indicate that atmospheric diffusion is at play
in our sample, with all of our hot subdwarfs at 25,000 to 50,000 K exhibiting
large enhancements of the Fe-peak elements. The hottest subdwarfs in our BHk
sample may be pulsators, given that they fall in the temperature range of
newly-discovered pulsating subdwarfs in omega Cen. In addition to the normal
hot HB and BHk stars, we obtain spectra of 5 blue HB stars, a post-HB star, and
3 unclassified stars with unusually blue UV colors.
The Astrophysical Journal 01/2012; 748(2). · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Blue hook stars are a class of subluminous extreme horizontal branch stars
that were discovered in UV images of the massive globular clusters omega Cen
and NGC 2808. These stars occupy a region of the HR diagram that is unexplained
by canonical stellar evolution theory. Using new theoretical evolutionary and
atmospheric models, we have shown that the blue hook stars are very likely the
progeny of stars that undergo extensive internal mixing during a late
helium-core flash on the white dwarf cooling curve. This "flash mixing"
produces hotter-than-normal EHB stars with atmospheres significantly enhanced
in helium and carbon. The larger bolometric correction, combined with the
decrease in hydrogen opacity, makes these stars appear subluminous in the
optical and UV. Flash mixing is more likely to occur in stars born with a high
helium abundance, due to their lower mass at the main sequence turnoff. For
this reason, the phenomenon is more common in those massive globular clusters
that show evidence for secondary populations enhanced in helium. However, a
high helium abundance does not, by itself, explain the presence of blue hook
stars in massive globular clusters. Here, we present new observational evidence
for flash mixing, using recent HST observations. These include UV
color-magnitude diagrams of six massive globular clusters and far-UV
spectroscopy of hot subdwarfs in one of these clusters (NGC 2808).
[Show abstract][Hide abstract] ABSTRACT: Recently, the understanding of globular clusters (GCs) has drastically changed owing to the improvement in spectroscopic and photometric observations, which have shown that not all GCs could be considered simple stellar populations. Whilst spectroscopic studies have shown variations of some light elements in different degrees in all observed GCs (e.g., O-Na anticorrelation, Carretta et al. 2009), photometric studies have shown splits in some phases of color-magnitude diagrams (CMDs), where the triple main sequence (MS) detected in NGC 2808 has been attributed to differences in the helium abundance (DeltaY, Piotto et al. 2007), which cannot be measured with enough precision in GCs. In this contribution, we show the other effects that must be observed in CMDs of GCs if DeltaY is real.
Astronomy and Astrophysics 10/2011; · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Moderately high resolution spectra of hot HB stars in the globular cluster omega Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. The spectroscopic data were obtained in 2005 (4 observations) and in 2006 (5 observations) in Service Mode using the MEDUSA mode of the multi-object fiber spectrograph FLAMES + GIRAFFE on the UT2 Telescope of the VLT. We used the low spectroscopic resolution mode with the spectral range 3964Å-4567Å (LR2, R=6400) and observed spectra for a total of 109 blue hook and canonical blue HB/EHB star candidates (see Table 1) and for 17 sky background positions. Each observation had an exposure time of 2550s to keep the total execution time of the observing block shorter than one hour. (3 data files).
[Show abstract][Hide abstract] ABSTRACT: We present HST/ACS observations of RR Lyrae variable stars in six ultra deep
fields of the Andromeda galaxy (M31), including parts of the halo, disk, and
giant stellar stream. Past work on the RR Lyrae stars in M31 has focused on
various aspects of the stellar populations that make up the galaxy's halo,
including their distances and metallicities. This study builds upon this
previous work by increasing the spatial coverage (something that has been
lacking in previous studies) and by searching for these variable stars in
constituents of the galaxy not yet explored. Besides the 55 RR Lyrae stars we
found in our initial field located 11kpc from the galactic nucleus, we find
additional RR Lyrae stars in four of the remaining five ultra deep fields as
follows: 21 in the disk, 24 in the giant stellar stream, 3 in the halo field
21kpc from the galactic nucleus, and 5 in one of the halo fields at 35kpc. No
RR Lyrae were found in the second halo field at 35kpc. The RR Lyrae populations
of these fields appear to mostly be of Oosterhoff I type, although the 11kpc
field appears to be intermediate or mixed. We will discuss the properties of
these stars including period and reddening distributions. We calculate
metallicities and distances for the stars in each of these fields using
different methods and compare the results, to an extent that has not yet been
done. We compare these methods not just on RR Lyrae stars in our M31 fields,
but also on a data set of Milky Way field RR Lyrae stars.
The Astronomical Journal 03/2011; · 4.97 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Recent high precision photometric observations of several globular clusters have revealed that these simple stellar populations may not be as simple as we once believed. Some globular clusters have shown interesting features in their color-magnitude diagrams indicating the presence of multiple populations. Recent HST observations of the massive globular cluster NGC 2808 have revealed the presence of three main sequences. We present new observations of NGC 2808 using the new WFC3 camera on HST. Observations are in five photometric bands covering a large wavelength range from the UVIS to the IR channels of WFC3. Moreover, these observations extend below the "turn down" in the lower main sequence (as seen in the IR CMD) and therefore reach deeper than past data sets on this cluster. We discuss various model fits and compare with other observed CMDs of other globular clusters to explore the origin of these multiple sequences.
[Show abstract][Hide abstract] ABSTRACT: UV observations of some massive globular clusters have revealed a significant population of stars hotter and fainter than the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either by the late hot flasher scenario where stars experience the helium flash while on the white dwarf cooling curve or by the progeny of the helium-enriched sub-population postulated to exist in some clusters. Previous spectroscopic analyses of blue hook stars in omega Cen and NGC 2808 support the late hot flasher scenario, but the stars contain much less helium than expected and the predicted C and N enrichment cannot be verified. We compare the observed effective temperatures, surface gravities, helium abundances, and carbon line strengths (where detectable) of our targets stars with the predictions of the two aforementioned scenarios. Moderately high resolution spectra of hot HB stars in the globular cluster omega Cen were analysed for radial velocity variations, atmospheric parameters, and abundances using LTE and non-LTE model atmospheres. We find no evidence of close binaries among our target stars. All stars below 30,000K are helium-poor and very similar to HB stars observed in that temperature range in other globular clusters. In the temperature range 30,000K to 50,000K, we find that 28% of our stars are helium-poor (log(He/H) < -1.6), while 72% have roughly solar or super-solar helium abundance (log(He/H) >= -1.5). We also find that carbon enrichment is strongly correlated with helium enrichment, with a maximum carbon enrichment of 3% by mass. A strong carbon enrichment in tandem with helium enrichment is predicted by the late hot flasher scenario, but not by the helium-enrichment scenario. We conclude that the helium-rich HB stars in omega Cen cannot be explained solely by the helium-enrichment scenario invoked to explain the blue main sequence. Comment: 11 pages, 8 figures, uses aa.cls (included), A&A in press
Astronomy and Astrophysics 09/2010; · 4.48 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present new Hubble Space Telescope ultraviolet color-magnitude diagrams of five massive Galactic globular clusters: NGC 2419, NGC 6273, NGC 6715, NGC 6388, and NGC 6441. These observations were obtained to investigate the "blue hook" (BH) phenomenon previously observed in UV images of the globular clusters ω Cen and NGC 2808. Blue hook stars are a class of hot (approximately 35,000 K) subluminous horizontal branch stars that occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. By coupling new stellar evolution models to appropriate non-LTE synthetic spectra, we investigate various theoretical explanations for these stars. Specifically, we compare our photometry to canonical models at standard cluster abundances, canonical models with enhanced helium (consistent with cluster self-enrichment at early times), and flash-mixed models formed via a late helium-core flash on the white dwarf cooling curve. We find that flash-mixed models are required to explain the faint luminosity of the BH stars, although neither the canonical models nor the flash-mixed models can explain the range of color observed in such stars, especially those in the most metal-rich clusters. Aside from the variation in the color range, no clear trends emerge in the morphology of the BH population with respect to metallicity.
The Astrophysical Journal 07/2010; 718(2):1332. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: Images were obtained using the Wide Field Camera on the ACS, in the F606W (broad V) and F814W (I) filters. The characteristics of the six fields observed are summarized in Table 1. (7 data files).
[Show abstract][Hide abstract] ABSTRACT: Globular clusters have long been considered the closest approximation to a physicist's laboratory in astrophysics, and as such a near-ideal laboratory for (low-mass) stellar evolution. However, recent observations have cast a shadow on this long-standing paradigm, suggesting the presence of multiple populations with widely different abundance patterns, and -- crucially -- with widely different helium abundances as well. In this review we discuss which features of the Hertzsprung-Russel diagram may be used as helium abundance indicators, and present an overview of available constraints on the helium abundance in globular clusters. Comment: 12 pages, 6 figures. Invited review, to appear in the proceedings of IAU Symp. 266 (ed. R. de Grijs & J. R. D. Lepine)
Proceedings of the International Astronomical Union 10/2009;
[Show abstract][Hide abstract] ABSTRACT: Using the Advanced Camera for Surveys on the Hubble Space Telescope, we have obtained deep optical images reaching well below the oldest main-sequence turnoff in six fields of the Andromeda Galaxy. The fields fall at four positions on the southeast minor axis, one position in the giant stellar stream, and one position on the northeast major axis. These data were obtained as part of three large observing programs designed to probe the star formation history of the stellar population in various structures of the galaxy. In this paper, we present the images, catalogs, and artificial star tests for these observing programs as a supplement to the analyses published previously. These high-level science products are also archived at the Multimission Archive at the Space Telescope Science Institute.
The Astrophysical Journal Supplement Series 08/2009; 184(1):152. · 14.14 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: It has recently been suggested that the presence of multiple populations showing various amounts of helium enhancement is the rule, rather than the exception, among globular star clusters. An important prediction of this helium enhancement scenario is that the helium-enhanced blue horizontal branch (HB) stars should be brighter than the red HB stars which are not helium-enhanced. In this Letter, we test this prediction in the case of the Galactic globular cluster M3 (NGC 5272), for which the helium-enhancement scenario predicts helium enhancements of > 0.02 in virtually all blue HB stars. Using high-precision Stroemgren photometry and spectroscopic gravities for blue HB stars, we find that any helium enhancement among most of the cluster's blue HB stars is very likely less than 0.01, thus ruling out the much higher helium enhancements that have been proposed in the literature. Comment: 6 pages, 4 figures. ApJ (Letters), in press
The Astrophysical Journal 02/2009; · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: An echelle spectrogram (R = 30,000) of the 2300-3100 Å region in the ultraviolet spectrum of the F8 V star 9 Comae is presented. The observation is used to calibrate features in the mid-ultraviolet spectra of similar stars according to age and metal content. In particular, the spectral break at 2640 Å is interpreted using the spectral synthesis code SYNSPEC. We use this feature to estimate the time since the last major star formation episode in the early-type galaxy LBDS 53W091 at redshift z=1.55, whose rest-frame mid-ultraviolet spectrum, observed with the Keck Telescope, is dominated by the flux from similar stars that are at or near the main-sequence turnoff in that system (Spinrad et al.). Our result, 1 Gyr if the flux-dominating stellar population has a metallicity twice solar, or 2 Gyr for a more plausible solar metallicity, is significantly lower than the previous estimate and thereby relaxes constraints on cosmological parameters that were implied by the earlier work.
The Astrophysical Journal 01/2009; 492(2):L131. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a color-magnitude diagram (CMD) for a minor-axis field in the halo of the Andromeda galaxy (M31), 51' (11 kpc) from the nucleus. These observations, taken with the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope, are the deepest optical images yet obtained, attaining 50% completeness at mV = 30.7 mag. The CMD, constructed from ~3 × 105 stars, reaches more than 1.5 mag fainter than the old main-sequence turnoff. Our analysis is based on direct comparisons to ACS observations of four globular clusters through the same filters, as well as χ2 fitting to a finely spaced grid of calibrated stellar-population models. We find that the M31 halo contains a major (~30% by mass) intermediate-age (6-8 Gyr) metal-rich ([Fe/H] > -0.5) population, as well as a significant globular cluster age (11-13.5 Gyr) metal-poor population. These findings support the idea that galaxy mergers played an important role in the formation of the M31 halo.
The Astrophysical Journal 12/2008; 592(1):L17. · 6.28 Impact Factor
[Show abstract][Hide abstract] ABSTRACT: We present a color-magnitude diagram (CMD) for a field in the giant tidal stream of the Andromeda galaxy (M31). These observations, taken with the Advanced Camera for Surveys on the Hubble Space Telescope, are 50% complete at V ≈ 30 mag, reaching 1 mag below the oldest main-sequence turnoff. Striking similarities between the stream and a previous spheroid CMD imply that they have very similar age and metallicity distributions, but present something of an enigma; we speculate on possible interpretations of this result, but we note that none are without problems. Distinct multiple turnoffs, as might be expected from pulses of star formation caused by interaction with Andromeda, are not apparent in the stream CMD. Star formation in both fields lasted about 6 billion years, building up to relatively high metallicities and being largely complete 6 billion years ago. The close similarity of the spheroid and stream suggests that both may have derived from the same event; it would be worth exploring to what extent stars in these structures are the remnants of a disk galaxy that interacted with M31 or even were disrupted from the M31 disk itself by the interaction.
The Astrophysical Journal 12/2008; 636(2):L89. · 6.28 Impact Factor