What we would like to know about extreme horizontal branch stars in globular clusters

Bidin, C. Moni; Piotto, G.

doi:10.1007/s10509-010-0391-1

Cited by 8 publications

(6 citation statements)

References 34 publications

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“…Evolutionary models can produce such objects when the mass loss on the RGB is artificially increased 1 , but the physical mechanisms able to produce the necessary strong mass loss are not well understood (D'Cruz et al 1996;Brown et al 2001). Identifying a suitable mechanism for enhanced mass loss in single stars is especially important when considering the formation of EHB stars in globular clusters, where binaries are notoriously rare (Moni Bidin & Piotto 2010). So far, only one binary candidate has been spectroscopically confirmed in NGC 6752 (Moni Bidin et al 2015).…”

Section: Introductionmentioning

confidence: 99%

Shotglas

Latour

Randall²,

Calamida

et al. 2018

A&A

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The presence of extreme horizontal branch (EHB) and blue hook stars in some Galactic globular clusters (GGCs) constitutes one of the remaining mysteries of stellar evolution. While several evolutionary scenarios have been proposed to explain the characteristics of this peculiar population of evolved stars, their observational verification has been limited by the availability of spectroscopic data for a statistically significant sample of such objects in any single GGC. We recently launched the SHOTGLAS project with the aim of providing a comprehensive picture of this intriguing stellar population in terms of spectroscopic properties for all readily accessible GGCs hosting an EHB. In this first paper, we focus on ω Cen, a peculiar, massive GGC that hosts multiple stellar populations. We use non-LTE model atmospheres to derive atmospheric parameters (Teff, log g and N(He)/N(H)) and spectroscopic masses for 152 EHB stars in the cluster. This constitutes the largest spectroscopic sample of EHB stars ever analyzed in a GGC and represents ≈20% of the EHB population of ω Cen. We also search for close binaries among these stars based on radial velocity variations. Our results show that the EHB population of ω Cen is divided into three spectroscopic groups that are very distinct in the Teff − helium abundance plane. The coolest sdB-type stars (Teff ≲ 30 000 K) have a hydrogen-rich atmosphere, populate the theoretical EHB region in the Teff − log g plane, and form 26% of our sample. The hottest sdO-type stars (Teff ≳ 42 000 K) make up 10% of the sample, have a hydrogen-rich atmosphere and are thought to be in a post-EHB evolutionary phase. The majority of our sample is found at intermediate temperatures and consists of sdOB stars that have roughly solar or super-solar atmospheric helium abundances. It is these objects that constitute the blue hook at V > 18.5 mag in the ω Cen color-magnitude diagram. Interestingly, the helium-enriched sdOBs do not have a significant counterpart population in the Galactic field, indicating that their formation is dependent on the particular environment found in ω Cen and other select GGCs. Another major difference between the EHB stars in ω Cen and the field is the fraction of close binaries. From our radial velocity survey we identify two binary candidates, however no orbital solutions could be determined. We estimate an EHB close binary fraction of ≈5% in ω Cen. This low fraction is in line with findings for other GGCs, but in sharp contrast to the situation in the field, where around 50% of the sdB stars reside in close binaries. Finally, the mass distribution derived is very similar for all three spectroscopic groups, however the average mass (0.38 M⊙) is lower than that expected from stellar evolution theory. While this mass conundrum has previously been noted for EHB stars in ω Cen, it so far appears to be unique to that cluster.

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Section: Introductionmentioning

confidence: 99%

Shotglas

Latour

Randall²,

Calamida

et al. 2018

A&A

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“…Horizontal branch (HB) stars in Galactic globular clusters (GCs) are old, low-initial mass (0.7-0.9 M ⊙ ) stars currently burning helium in their core (Hoyle & Schwarzschild 1955;Faulkner 1966). Owing to their complexity, some aspects of the formation and internal structure of HB stars have been a matter of debate for decades (see Catelan 2009;Moni Bidin & Piotto 2010, for recent reviews). For instance, the blue extension of the HB varies among clusters, a fact partly associated with the metallicity (Sandage & Wallerstein 1960) but not entirely explained by it (Sandage & Wildey 1967;van den Bergh 1967).…”

Section: Introductionmentioning

confidence: 99%

Spectroscopy of blue horizontal branch stars in NGC 6656 (M 22)

Salgado¹,

Bidin²,

Villanova³

et al. 2013

A&A

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Context. Recent investigations revealed very peculiar properties of blue horizontal branch (HB) stars in ω Centauri, which show anomalously low surface gravity and mass compared to other clusters and to theoretical models. ω Centauri, however, is a very unusual object, hosting a complex mix of multiple stellar populations with different metallicity and chemical abundances. Aims. We measured the fundamental parameters (temperature, gravity, and surface helium abundance) of a sample of 71 blue HB stars in M 22, with the aim of clarifying if the peculiar results found in ω Cen are unique to this cluster. M 22 also hosts multiple subpopulations of stars with a spread in metallicity, analogous to ω Cen. Methods. The stellar parameters were measured on low-resolution spectra fitting the Balmer and helium lines with a grid of synthetic spectra. From these parameters, the mass and reddening were estimated. Results. Our results on the gravities and masses agree well with theoretical expectations, matching the previous measurements in three "normal" clusters. The anomalies found in ω Cen are not observed among our stars. A mild mass underestimate is found for stars hotter than 14 000 K, but an exact analogy with ω Cen cannot be drawn. We measured the reddening in the direction of M 22 with two independent methods, finding E(B − V) = 0.35 ± 0.02 mag, with semi-amplitude of the maximum variation Δ(E(B − V)) = 0.06 mag, and an rms intrinsic dispersion of σ(E(B − V)) = 0.03 mag.

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“…Moni Bidin & Piotto (2010) suggest that the star M5865 in globular cluster NGC 6752 is a hot subdwarf + G-K MS binary with a period of order 5 d, but details have not been published.…”

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confidence: 99%

Two New Long-Period Hot Subdwarf Binaries With Dwarf Companions

Barlow

Liss

Wade

et al. 2013

ApJ

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Hot subdwarf stars with F-K main sequence binary companions have been known for decades, but the first orbital periods for such systems were published just recently. Current observations suggest that most have long periods, on the order of years, and that some are or once were hierarchical triple systems. As part of a survey with the Hobby-Eberly Telescope, we have been monitoring the radial velocities of several composite-spectra binaries since 2005 in order to determine their periods, velocities, and eccentricities. Here we present observations and orbital solutions for two of these systems, PG 1449+653 and PG 1701+359. Similar to the other sdB+F/G/K binaries with solved orbits, their periods are long, 909 d and 734 d, respectively, and pose a challenge to current binary population synthesis models of hot subdwarf stars. Intrigued by their relatively large systemic velocities, we also present a kinematical analysis of both targets and find that neither is likely a member of the Galactic thin disk.

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What we would like to know about extreme horizontal branch stars in globular clusters

Cited by 8 publications

References 34 publications

Shotglas

Shotglas

Spectroscopy of blue horizontal branch stars in NGC 6656 (M 22)

Two New Long-Period Hot Subdwarf Binaries With Dwarf Companions

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