Blue Straggler Stars in the Unusual Globular Cluster NGC 63881

Dalessandro, E.; Lanzoni, B.; Ferraro, F. R.; Rood, Robert T.; Milone, A. P.; Piotto, G.; Valenti, E.

doi:10.1086/529028

Cited by 78 publications

(106 citation statements)

References 47 publications

(52 reference statements)

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“…The source distances are obtained from the following references (given in order) : Chenevez et al (2012), Ortolani et al (2007), Harris (1996, using the updated version of 2010), Ortolani et al (1994), Dalessandro et al (2008) and Galloway & Cumming (2006). † No best-fit N H was calculated for SAX J1808.4−3658 (see Appendix A).…”

Section: Resultsmentioning

confidence: 99%

Very hard states in neutron star low-mass X-ray binaries

Parikh

Wijnands

Degenaar

et al. 2017

Monthly Notices of the Royal Astronomical Society

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We report on unusually very hard spectral states in three confirmed neutron-star low-mass X-ray binaries (1RXS J180408.9−342058, EXO 1745−248, and IGR J18245−2452) at a luminosity between ∼ 10 36−37 erg s −1. When fitting the Swift X-ray spectra (0.5 -10 keV) in those states with an absorbed power-law model, we found photon indices of Γ ∼ 1, significantly lower than the Γ = 1.5 -2.0 typically seen when such systems are in their so called hard state. For individual sources very hard spectra were already previously identified but here we show for the first time that likely our sources were in a distinct spectral state (i.e., different from the hard state) when they exhibited such very hard spectra. It is unclear how such very hard spectra can be formed; if the emission mechanism is similar to that operating in their hard states (i.e., up-scattering of soft photons due to hot electrons) then the electrons should have higher temperatures or a higher optical depth in the very hard state compared to those observed in the hard state. By using our obtained Γ as a tracer for the spectral evolution with luminosity, we have compared our results with those obtained by Wijnands et al. (2015). We confirm their general results in that also our sample of sources follow the same track as the other neutron star systems, although we do not find that the accreting millisecond pulsars are systematically harder than the non-pulsating systems.

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Section: Resultsmentioning

confidence: 99%

Very hard states in neutron star low-mass X-ray binaries

Parikh

Wijnands

Degenaar

et al. 2017

Monthly Notices of the Royal Astronomical Society

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“…-M80, M13, M3 -This is the popular cluster triplet at intermediate metallicity with different HB morphologies, including red clumps, gaps, and blue tails (see, e.g., Ferraro et al 1997Ferraro et al , 1998Dalessandro et al 2013a (Rich et al 1997;Pritzl et al 2001Pritzl et al , 2002Pritzl et al , 2003Busso et al 2007;Dalessandro et al 2008). …”

Section: Observations Data Reduction and Photometric Analysismentioning

confidence: 99%

“…Some quantitative investigations of the HB morphology of the massive GCs NGC 2808, NGC 6388, and NGC 6441 HB, were recently performed (Busso et al 2007;Dalessandro et al 2008Dalessandro et al , 2011Brown et al 2010).…”

Section: Mass Loss Rates and Hb Morphologymentioning

confidence: 99%

An empirical mass-loss law for Population II giants from theSpitzer-IRAC survey of Galactic globular clusters

Origlia

Ferraro

Fabbri

et al. 2014

A&A

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Aims. The main aim of the present work is to derive an empirical mass-loss (ML) law for Population II stars in first and second ascent red giant branches. Methods. We used the Spitzer InfraRed Array Camera (IRAC) photometry obtained in the 3.6-8 μm range of a carefully chosen sample of 15 Galactic globular clusters spanning the entire metallicity range and sampling the vast zoology of horizontal branch (HB) morphologies. We complemented the IRAC photometry with near-infrared data to build suitable color-magnitude and color-color diagrams and identify mass-losing giant stars. Results. We find that while the majority of stars show colors typical of cool giants, some stars show an excess of mid-infrared light that is larger than expected from their photospheric emission and that is plausibly due to dust formation in mass flowing from them. For these stars, we estimate dust and total (gas + dust) ML rates and timescales. We finally calibrate an empirical ML law for Population II red and asymptotic giant branch stars with varying metallicity. We find that at a given red giant branch luminosity only a fraction of the stars are losing mass. From this, we conclude that ML is episodic and is active only a fraction of the time, which we define as the duty cycle. The fraction of mass-losing stars increases by increasing the stellar luminosity and metallicity. The ML rate, as estimated from reasonable assumptions for the gas-to-dust ratio and expansion velocity, depends on metallicity and slowly increases with decreasing metallicity. In contrast, the duty cycle increases with increasing metallicity, with the net result that total ML increases moderately with increasing metallicity, about 0.1 M every dex in [Fe/H]. For Population II asymptotic giant branch stars, we estimate a total ML of ≤0.1 M , nearly constant with varying metallicity.

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“…The general rule is that metal-rich systems have red HBs, while in the metal-poor ones stars are distributed on average at higher effective temperatures. However several exceptions to this general trend are observed: remarkable cases are those of NGC 6388 and NGC 6441 (Rich et al 1997), which despite their metallicity ([Fe/H]∼ −0.6) show some of the bluest HBs known in GGCs (Busso et al 2007;Dalessandro et al 2008 (Freeman & Norris 1981) and possibly a third parameter in order to explain the HB distributions in all GGCs. What we can call now the i-th parameter problem is still a hot topic, as stressed by several authors, we recall the reader to Catelan 2009 for a nice review (see also Dotter et al 2010 andGratton et al 2010).…”

Section: Introductionmentioning

confidence: 90%

Old stellar systems in UV: resolved and integrated properties

Dalessandro¹

2014

Astrophys Space Sci

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The UV properties of old stellar populations have been subject of intense scrutiny from the late sixties, when the UV-upturn in early type galaxies was first discovered. Because of their proximity and relative simplicity, Galactic globular clusters (GGCs) are ideal local templates to understand how the integrated UV light is driven by hot stellar populations, primarily horizontal branch stars and their progeny. Our understanding of such stars is still plagued by theoretical uncertainties, which are partly due to the absence of an accurate, comprehensive, statistically representative homogeneous data-set. To move a step forward on this subject, we have combined the HST and GALEX capabilities and collected the largest data-base ever obtained for GGCs in UV. This data-base is best suited to provide insights on the HB second parameter problem and on the first stages of GCs formation and chemical evolution and to understand how they are linked to the observed properties of extragalactic systems.

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Blue Straggler Stars in the Unusual Globular Cluster NGC 63881

Cited by 78 publications

References 47 publications

Very hard states in neutron star low-mass X-ray binaries

Very hard states in neutron star low-mass X-ray binaries

An empirical mass-loss law for Population II giants from theSpitzer-IRAC survey of Galactic globular clusters

Old stellar systems in UV: resolved and integrated properties

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