arXiv subjects

S. Moehler

Publications and source records attributed to S. Moehler.

At least 37 records · Page 2Linked to original sources

arXiv2009

A lack of close binaries among hot horizontal branch stars in globular clusters - M80 and NGC5986

Recent investigations have revealed a surprising lack of close binaries among extreme horizontal branch (EHB) stars in the globular cluster NGC6752, at variance with the analogous sdB field stars. Another puzzling result concerns the derived spectroscopic masses for some EHB stars. The present paper extends our study of NGC6752 to M80 and NGC5986. Twenty-one horizontal branch stars (out of which 5 EHBs) in NGC5986 and 31 in M80 (11 EHBs) were observed during four consecutive nights. We measured radial velocity variations, temperatures, gravities, and helium abundances. By means of a statistical analysis, we detected one EHB close binary candidate per cluster. In M80, the best estimate of the close binary EHB fraction is f=12%, and even the lowest estimate of the binary fraction among field sdB stars can be ruled out within a 90% confidence level. Because of the small observed sample, no strong conclusions can be drawn on the close EHB binary fraction for NGC5986, although our best estimate is rather low (f=25%). For the discrepancy in spectroscopic derived masses with theoretical models observed in NGC6752, our analysis of M80 EHB stars shows a similar trend. For the first time, we report a clear trend in surface helium abundance with temperature. Our results show that the deficiency of close binaries among EHB stars is now confirmed in two, and possibly three, globular clusters. This feature is therefore not a peculiarity of NGC6752. Our analysis also proves that the strangely high spectroscopic masses among EHB stars are now confirmed in at least a second cluster. Our results confirm that f could be a function of the age of the sdB star population, but we find that recent models have some problem reproducing all observations.

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arXiv2007

Binaries among Extreme Horizontal Branch Stars in Globular Clusters

Following a review of our present knowledge about blue subdwarf stars in globular clusters, we present an overview of the results of searches for close binaries among these stars, including results previously published in the literature and reporting recent and preliminary results of new data. Previous investigations revealed a lack of close systems in NGC 6752, which we confirm with new, more extensive observations. Our estimate of the close binary fraction in this cluster is only 4%. From a review of the relevant literature, there are indications that a low close binary fraction among extreme horizontal branch (EHB) stars is a common feature in globular star clusters. On the other hand, the field EHB population shows evidence of a remarkably high binary fraction. Such a difference among globular cluster and field populations, although not yet explained in detail on the basis of theoretical models, must necessarily be related to different formation histories for EHB stars in the field and in clusters. In this framework, preliminary results indicate that close systems could be relatively common in the peculiar globular cluster NGC 2808, although the sample of studied stars is still small. This would imply that not all clusters share the same behavior, as far as EHB star formation is concerned. We briefly explore possible explanations for these results.

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arXiv2007

The Hottest Horizontal-Branch Stars in omega Centauri - Late Hot Flasher vs. Helium Enrichment

UV observations of some massive globular clusters uncovered a significant population of very hot stars below the hot end of the horizontal branch (HB), the so-called blue hook stars. This feature might be explained either as results of 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 recently postulated to exist in some clusters. Moderately high resolution spectra of stars at the hot end of the blue HB in omega Cen were analysed for atmospheric parameters and abundances using LTE and Non-LTE model atmospheres. In the temperature range 30,000K to 50,000K we find that 35% of our stars are helium-poor (log(n_He/n_H) < -2), 51% have solar helium abundance within a factor of 3 (-1.5 <= log(n_He/n_H) <= -0.5) and 14% are helium-rich (log(n_He/n_H)> -0.4). We also find carbon enrichment in step with helium enrichment, with a maximum carbon enrichment of 3% by mass. At least 14% of the hottest HB stars in omega Cen show helium abundances well above the highest predictions from the helium enrichment scenario (Y = 0.42 corresponding to log(n_He/n_H) ~ -0.74). In addition, the most helium-rich stars show strong carbon enrichment as predicted by the late hot flasher 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. (Abridged)

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arXiv2007

Spectroscopy of horizontal branch stars in NGC6752 - Anomalous results on atmospheric parameters and masses

We used the ESO VLT-FORS2 facility to collect low-resolution spectra of 51 targets distributed along the Horizontal Branch. We determined atmospheric parameters by comparison with theoretical models through standard fitting routines, and masses by basic equations. Results are in general in good agreement with previous works, although not always with theoretical expectations for cooler stars (Teff<15000 K). The calculated color excess is systematically lower than literature values, pointing towards a possible underestimation of effective temperatures. Moreover, we find two groups of stars at Teff=14000 K and at Teff=27000$ K that present anomalies with respect to the general trend and expectations. We suppose that the three peculiar bright stars at Teff=14000 K are probably affected by an enhanced stellar wind. For the eight Extreme Horizontal Branch stars at Teff=27000 K which show unusually high masses we find no plausible explanation. While most of our results agree well with the predictions of standard horizontal branch evolution, we still have problems with the low masses we derive in certain temperature ranges. We believe that Kurucz ATLAS9 LTE model atmospheres with solar-scaled abundances are probably inadequate for these temperature ranges. Concerning the group of anomalous stars at Teff=27000 K, a Kolmogorov-Smirnov test indicates that there is only an 8.4% probability that these stars are randomly drawn from the general distribution in the color-magnitude diagram. This is not conclusive but points out that these stars could be both (and independently) spectroscopically and photometrically peculiar with respect to the general Extreme Horizontal Branch population.

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arXiv2007

Good News for MOS, MXU & Co. - The New Spectroscopic Pipeline for the FORSes

Since October 1, 2006, spectroscopic data from the two FORS instruments have been reduced with a new pipeline, which is based on a bottom-up calibration approach. I give a short description of the pipeline and discuss first experiences with automatic data reduction using this software, which has significantly increased the percentage of processed data for both instruments. I will also describe possible new options for Quality Control.

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arXiv2007

Na-O Anticorrelation and Horizontal Branches. V. The Na-O anticorrelation in NGC 6441 from Giraffe spectra

We present an analysis of FLAMES-Giraffe spectra for several bright giants in NGC 6441, to investigate the presence and extent of the Na-O anticorrelation in this anomalous globular cluster. The field of NGC 6441 is very crowded, with severe contamination by foreground (mainly bulge) field stars. Appropriate membership criteria were devised to identify a group of 25 likely cluster members among the about 130 stars observed. Combined with the UVES data obtained with the same observations, high dispersion abundance analyses are now available for a total of 30 stars in NGC 6441, 29 of them having data for both O and Na. The spectra were analyzed by a standard line analysis procedure; care was taken to minimize the impact of the differential interstellar reddening throughout the cluster, and to extract reliable information from crowded, and moderately high S/N (30-70), moderately high resolution (R ~ 23,000) spectra. NGC 6441 is very metal-rich ([Fe/H]=$-0.34\pm 0.02\pm0.04$ dex). There is no clear sign of star-to-star scatter in the Fe-peak elements. The alpha-elements Mg, Si, Ca, and Ti are overabundant by rather large factors, suggesting that the cluster formed from material enriched by massive core collapse SNe. The O-Na anticorrelation is well defined, with about 1/4 of the stars being Na-rich and O-poor. One of the stars is a Ba-rich and moderately C-rich star. The distribution of [Na/O] ratios among RGB stars in NGC 6441 appears similar to the distribution of colors of stars along the horizontal branch. The fraction of Na-poor, O-rich stars found in NGC 6441 agrees well with that of stars on the red horizontal branch of this cluster (in both cases about 80%), with a sloping distribution toward lower values of [O/Na] (among RGB stars and bluer colors (among HB stars).

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arXiv2007

Na-O Anticorrelation and HB. VI. The chemical composition of the peculiar bulge globular cluster NGC 6388

We present the LTE abundance analysis of high resolution spectra for red giant stars in the peculiar bulge globular cluster NGC 6388. Spectra of seven members were taken using the UVES spectrograph at the ESO VLT2 and the multiobject FLAMES facility. We exclude any intrinsic metallicity spread in this cluster: on average, [Fe/H]=-0.44+/-0.01+/-0.03 dex on the scale of the present series of papers, where the first error bar refers to individual star-to-star errors and the second is systematic, relative to the cluster. Elements involved in H-burning at high temperatures show large spreads, exceeding the estimated errors in the analysis. In particular, the pairs Na and O, Al and Mg are anticorrelated and Na and Al are correlated among the giants in NGC 6388, the typical pattern observed in all galactic globular clusters studied so far. Stars in NGC 6388 shows an excess of alpha-process elements, similar to the one found in the twin bulge cluster NGC 6441. Mn is found underabundant in NGC 6388, in agreement with the average abundance ratio shown by clusters of any metallicity. Abundances of neutron-capture elements are homogeneously distributed within NGC 6388; the [Eu/Fe] ratio stands above the value found in field stars of similar metallicity.

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arXiv2006

Discovery of Carbon/Oxygen depleted Blue Straggler Stars in 47 Tucanae: the chemical signature of a mass-transfer formation process

We use high-resolution spectra obtained with the ESO Very Large Telescope to measure surface abundance patterns of 43 Blue Stragglers stars (BSS) in 47 Tuc. We discovered that a sub-population of BSS shows a significant depletion of Carbon and Oxygen with respect to the dominant population. This evidence would suggest the presence of CNO burning products on the BSS surface coming from a deeply peeled parent star, as expected in the case of mass-transfer process. This is the first detection of a chemical signature clearly pointing to a specific BSS formation process in a globular cluster.

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arXiv2006

Observations of the Hot Horizontal-Branch Stars in the Metal-Rich Bulge Globular Cluster NGC 6388 - Indications of Helium Enrichment and a Lesson in Crowded Field Spectroscopy

(abridged) The metal-rich bulge globular cluster NGC6388 shows a distinct blue horizontal-branch tail with a strong upward tilt in its colour-magnitude diagram. Several noncanonical scenarios have been proposed to explain these puzzling observations. In order to test the predictions of these scenarios, we have obtained medium resolution spectra to determine the atmospheric parameters of a sample of the blue HB stars in NGC 6388. Using the medium resolution spectra, we determine effective temperatures, surface gravities and helium abundances by fitting the observed Balmer and helium lines with appropriate theoretical stellar spectra. As we know the distance to the cluster, we can verify our results by determining masses for the stars. During the data reduction we took special care in subtracting the background, which is dominated by the overlapping spectra of cool stars. The physical parameters of the blue HB stars near 10,000 K support the helium pollution scenario. The low gravities and masses found for the hot blue tail stars, however, are probably caused by problems with the data reduction, most likely due to remaining background light in the spectra, which would affect the fainter hot blue tail stars much more strongly than the brighter cool blue tail stars. Our study of the hot blue tail stars in NGC 6388 illustrates the obstacles which are encountered when attempting to determine the atmospheric parameters of hot HB stars in very crowded fields using ground-based observations. We discuss these obstacles and offer possible solutions for future projects.

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arXiv2006

The lack of binaries among hot horizontal branch stars: M80 and NGC5986

We present preliminary results of a our search for close (period P < 10 days) binary systems among hot (Teff > 20000 K) horizontal branch stars in globular clusters M80 and NGC5986. We measured radial velocity variations of 11 targets in M80 and 5 in NGC5986, spectroscopically observed at medium resolution (R=4100) during four nights at VLT-FORS2. We found one close binary candidate in M80, showing statistically significative radial velocity variations, and no clear evidence in NGC5986. These results confirm the lack of close binaries among hot horizontal branch stars in globular clusters, as already found in NGC6752, at variance with what observed among hot subdwarfs in the field.

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arXiv2006

The lack of close binaries among hot horizontal branch stars in NGC6752

We present the results of a spectroscopic search for close binaries among horizontal branch (HB) stars in NGC6752. We used the ESO VLT-FORS2 instrument to obtain medium resolution (R=4100) spectra of 51 hot HB stars with 8000 < Teff < 32000 K during four consecutive nights. Eighteen of our targets are extreme horizontal branch (EHB) stars with Teff > 22000 K. Radial velocity variations were measured with cross-correlation techniques and we carefully evaluated the statistical and systematic errors associated with them. No close binary system has been detected among our 51 targets. The data corrected for instrumental effects indicate that the radial velocity variations are always below 15 km/s (3sigma level). From a statistical analysis of our results, we conclude that (at 95 confidence level) the fraction of binaries with a ~0.5 solar masses companion among EHB stars in NGC6752 is smaller than 20%. This empirical evidence sharply contrasts with what has been found for hot subdwarfs in the field, and opens new questions about the formation of EHB stars in globular clusters (and possibly in the field as well.

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arXiv2005

Spectroscopic search for binaries among EHB stars in globular clusters

We performed a spectroscopic search for binaries among hot Horizontal Branch stars in globular clusters. We present final results for a sample of 51 stars in NGC6752, and preliminary results for the first 15 stars analyzed in M80. The observed stars are distributed along all the HBs in the range 8000 < Teff < 32000 K, and have been observed during four nights. Radial velocity variations have been measured with the cross-correlation technique. We carefully analyzed the statistical and systematic errors associated with the measurements in order to evaluate the statistical significance of the observed variations. No close binary system has been detected, neither among cooler stars nor among the sample of hot EHB stars (18 stars with Teff > 22000 K in NGC6752). The data corrected for instrumental effects indicate that the radial velocity variations are always below the 3sigma level of ~15 km/s. These results are in sharp contrast with those found for field hot subdwarfs, and open new questions about the formation of EHB stars in globular clusters, and possibly of the field subdwarfs.

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arXiv2004

Horizontal Branch A- and B-type Stars in Globular Clusters

Globular clusters offer ideal laboratories to test the predictions of stellar evolution. When doing so with spectroscopic analyses during the 1990s, however, the parameters we derived for hot horizontal branch stars deviated systematically from theoretical predictions. The parameters of cooler, A-type horizontal branch stars, on the other hand, were consistent with evolutionary theories. In 1999, two groups independently suggested that diffusion effects might cause these deviations, which we verified subsequently. I will discuss these observations and analyses and their consequences for interpreting observations of hot horizontal branch stars.

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arXiv2004

Double degenerates and progenitors of supernovae type Ia

We report on systematic radial velocity surveys for white dwarf - white dwarf binaries (double degenerates - DDs) including SPY (ESO Supernovae Ia progenitor survey) recently carried out at the VLT. A large sample of DD will allow us to put strong constrains on the phases of close binary evolution of the progenitor systems and to perform an observational test of the DD scenario for supernovae of type Ia. We explain how parameters of the binaries can be derived from various methods. Results for a sample of DDs are presented and discussed.

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arXiv2004

Spectral Types and Masses of White Dwarfs in Globular Clusters

White dwarfs in globular clusters offer additional possibilities to determine distances and ages of globular clusters, provided their spectral types and masses are known. We therefore started a project to obtain spectra of white dwarfs in the globular clusters NGC 6397 and NGC 6752. All observed white dwarfs show hydrogen-rich spectra and are therefore classified as DA. Analysing the multi-colour photometry of the white dwarfs in NGC 6752 yields an average gravity of log g = 7.84 and 0.53 Msolar as the most probable average mass for globular cluster white dwarfs. Using this average gravity we try to determine independent temperatures by fitting the white dwarf spectra. While the stellar parameters determined from spectroscopy and photometry usually agree within the mutual error bars, the low resolution and S/N of the spectra prevent us from setting constraints stronger than the ones derived from the photometry alone. For the same reasons the white dwarf spectra obtained for NGC 6397 unfortunately do not provide an independent distance estimate of sufficient accuracy to distinguish between the long and short distance scale for globular clusters.

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arXiv2003

Spectroscopic Analyses of the Blue Hook Stars in NGC 2808: A More Stringent Test of the Late Hot Flasher Scenario

Recent UV observations of the globular cluster NGC2808 (Brown et al. 2001) show a significant population of hot stars fainter than the zero-age horizontal branch (``blue hook'' stars), which cannot be explained by canonical stellar evolution. Their results suggest that stars which experience unusually large mass loss on the red giant branch and which subsequently undergo the helium core flash while descending the white dwarf cooling curve could populate this region. Theory predicts that these ``late hot flashers'' should show higher temperatures than the hottest canonical horizontal branch (HB) stars and should have He- and C-rich atmospheres. As a test of this late hot flasher scenario, we have obtained and analysed medium resolution spectra of a sample of blue hook stars in NGC2808 to derive their atmospheric parameters. Using the same procedures, we have also re-analyzed our earlier spectra of the blue hook stars in omega Cen (Moehler et al. 2002) for comparison with the present results for NGC2808. The blue hook stars in these two clusters are both hotter (Teff > 35,000 K) and more helium-rich than canonical extreme HB stars in agreement with the late hot flasher scenario. Moreover, we find indications for C enhancement in the three most He-enriched stars in NGC2808. However, the blue hook stars still show some H in their atmospheres, perhaps indicating that some residual H survives a late hot flash and then later diffuses to the surface during the HB phase. We note that the presence of blue hook stars apparently depends mostly on the total mass of the globular cluster and not so much on its HB morphology.

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arXiv2003

Helium-rich EHB Stars in Globular Clusters

Recent UV observations of the most massive Galactic globular clusters show a significant population of hot stars below the zero-age HB (``blue hook'' stars), which cannot be explained by canonical stellar evolution. Stars which suffer unusually large mass loss on the red giant branch and thus experience the helium-core flash while descending the white dwarf cooling curve could populate this region. They should show higher temperatures than the hottest canonical HB stars and their atmospheres should be helium-rich and probably C/N-rich. We have obtained spectra of blue hook stars in omega Cen and NGC 2808 to test this possibility. Our analysis shows that the blue hook stars in these clusters reach effective temperatures well beyond the hot end of the canonical EHB and have higher helium abundances than canonical EHB stars. These results support the hypothesis that the blue hook stars arise from stars which ignite helium on the white dwarf cooling curve.

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arXiv2003

Hot HB stars in globular clusters - Physical parameters and consequences for theory. VI. The second parameter pair M 3 and M 13

We present the results of spectroscopic analyses of hot horizontal branch (HB) stars in M 13 and M 3, which form a famous ``second parameter'' pair. From the spectra and Stroemgren photometry we derived -- for the first time in M 13 -- atmospheric parameters (effective temperature and surface gravity). For stars with Stroemgren temperatures between 10,000 and 12,000 K we found excellent agreement between the atmospheric parameters derived from Stroemgren photometry and those derived from Balmer line profile fits. However, for cooler stars there is a disagreement in the parameters derived by the two methods, for which we have no satisfactory explanation. Stars hotter than 12,000 K show evidence for helium depletion and iron enrichment, both in M 3 and M 13. Accounting for the iron enrichment substantially improves the agreement with canonical evolutionary models, although the derived gravities and masses are still somewhat too low. This remaining discrepancy may be an indication that scaled-solar metal-rich model atmospheres do not adequately represent the highly non-solar abundance ratios found in blue HB stars affected by diffusion. We discuss the effects of an enhancement in the envelope helium abundance on the atmospheric parameters of the blue HB stars, as might be caused by deep mixing on the red giant branch or primordial pollution from an earlier generation of intermediate mass asymptotic giant branch stars.

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