arXiv subjects

C. Moni Bidin

Publications and source records attributed to C. Moni Bidin.

63 records · Page 4Linked 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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arXiv2008

Is a binary fraction-age relation responsible for the lack of EHB binaries in globular clusters?

The recently-discovered lack of close binaries, among extreme horizontal branch (EHB) stars in Galactic globular clusters, has thus far constituted a major puzzle, in view of the fact that blue subdwarf stars - the field counterparts of cluster EHB stars - are well-known to present a high binary fraction. In this Letter, we provide new results that confirm the lack of close EHB binaries in globular clusters, and present a first scenario to explain the difference between field and cluster EHB stars. First, in order to confirm that the lack of EHB binaries in globular clusters is a statistically robust result, we undertook a new analysis of 145 horizontal branch stars in NGC6752, out of which forty-one belong to the EHB. To search for radial-velocity variations as a function of time, we repeated high-resolution (R=18500) spectroscopy of all stars, four times during a single night of observations. We detected a single, hot (25000 K), radial-velocity variable star as a close-binary candidate. From these results, we estimate an upper-limit for the close (period P < 5 day) binary fraction f among NGC6752 EHB stars of 16% (95% confidence level), with the most probable value being f=4%. Thus our results clearly confirm the lack of close binaries among the hot HB stars in this cluster. We suggest that the confirmed discrepancy between the binary fractions for field and cluster EHB stars is the consequence of an f-age relation, with close binaries being more likely in the case of younger systems. We analyze theoretical and observational results available in the literature, which support this scenario. If so, an age difference between the EHB progenitors in the field and in clusters, the former being younger (on average) by up to several Gyr, would naturally account for the startling differences in binary fraction between the two populations.

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arXiv2007

The vertical velocity dispersion profile of the Galactic thick disk

We present the results of radial velocity measurements of 770 thick disk red giants toward the South Galactic Pole, vertically distributed from 0.5 kpc to 5 kpc with respect to the Galactic plane. We find a small gradient in the vertical velocity dispersion (sigma_W) of 3.8+/-0.8 km/s kpc. Even more noteworthy, our values of $σ_W$ are small compared to literature values: in the middle of the vertical height range we find sigma_W(z=2kpc)=30 km/s. We found no possible explanation for this small value of sigma_W in terms of sample contamination by thin disk stars, nor by wrong assumptions regarding the metallicity distribution and the derived distances.

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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

A Search for EHB Pulsators in the Globular Cluster NGC 6752

We describe the status of a project whose main goal is to detect variability along the extreme horizontal branch of the globular cluster NGC 6752. Based on Magellan 6.5m data, preliminary light curves are presented for some candidate variables. By combining our time-series data, we also produce a deep CMD of unprecedented quality for the cluster which reveals a remarkable lack of main sequence binaries, possibly pointing to a low primordial binary fraction.

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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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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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