Horizontal Branch Stars and the Ultraviolet Universe

Catelan, M.

doi:10.1007/978-0-387-87621-4_27

Cited by 5 publications

(5 citation statements)

References 80 publications

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“…Given the small fraction of the lifetime that stars live on the HB, it would require a significant low metallicity main sequence population to exist within these galaxies, which is explicitly ruled out by many studies determining the metallicities of such galaxies. The explanation of these UV upturn galaxies lies in the understanding gleaned from observations of the resolved stellar populations in high metallicity open clusters and socalled "second parameter" globular clusters (Catelan 2009;Peacock et al 2017).…”

Section: Hot Hb Stars As the Source Of The Uv Upturnmentioning

confidence: 99%

UV SEDs of early-type cluster galaxies: a new look at the UV upturn

Ali

Bremer

Phillipps

et al. 2018

Monthly Notices of the Royal Astronomical Society

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Using GALEX, UVOT and optical photometry, we explore the prevalence and strength of the UV-upturn in the spectra of quiescent early type galaxies in several nearby clusters. Even for galaxies with completely passive optical colours, there is a large spread in vacuum UV colour consistent with almost all having some UV upturn component. Combining GALEX and UVOT data below 3000Å, we generate for the first time comparatively detailed UV SEDs for Coma cluster galaxies. Fitting the UV upturn component with a blackbody, twenty six of these show a range of characteristic temperatures (10000-21000K) for the UV upturn population. Assuming a single temperature to explain GALEX-optical colours could underestimate the fraction of galaxies with UV upturns and mis-classify some as systems with residual star formation. The UV upturn phenomenon is not an exclusive feature found only in giant galaxies; we identify galaxies with similar (or even bluer) FUV −V colours to the giants with upturns over a range of fainter luminosities. The temperature and strength of the UV upturn are correlated with galaxy mass. Under the plausible hypothesis that the sources of the UV upturn are blue horizontal branch stars, the most likely mechanism for this is the presence of a substantial (between 4% and 20%) Helium rich (Y > 0.3) population of stars in these galaxies, potentially formed at z ∼ 4 and certainly at z > 2; this plausibly sets a lower limit of ∼ 0.3 − 0.8 × 10 10 M to the in situ stellar mass of ∼ L * galaxies at this redshift.

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Section: Hot Hb Stars As the Source Of The Uv Upturnmentioning

confidence: 99%

UV SEDs of early-type cluster galaxies: a new look at the UV upturn

Ali

Bremer

Phillipps

et al. 2018

Monthly Notices of the Royal Astronomical Society

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“…Carter et al (2011) found that the UVX depends more strongly on [α/Fe] than [Fe/H]. An abundance gradient in [α/Fe] could be expected if high amounts of star formation and supernovae increase the α-element enrichment of stars in the bulges of galaxies (Greggio & Renzini 1999;Bressan et al 1994;Catelan 2009). However, Saglia et al (2010) found no obvious gradient in [α/Fe] over the regions we analyze in M31.…”

Section: Galactic Metallicity Gradientmentioning

confidence: 99%

The Panchromatic Hubble Andromeda Treasury. I. Bright Uv Stars in the Bulge of M31

Rosenfield

Johnson

Girardi

et al. 2012

ApJ

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As part of the Panchromatic Hubble Andromeda Treasury (PHAT) multi-cycle program, we observed a 12 × 6.5 area of the bulge of M31 with the WFC3/UVIS filters F 275W and F 336W . From these data we have assembled a sample of ∼4000 UV-bright, old stars, vastly larger than previously available. We use updated Padova stellar evolutionary tracks to classify these hot stars into three classes: Post-AGB stars (P-AGB), Post-Early AGB (PE-AGB) stars and AGB-manqué stars. P-AGB stars are the end result of the asymptotic giant branch (AGB) phase and are expected in a wide range of stellar populations, whereas PE-AGB and AGB-manqué (together referred to as the hot post-horizontal branch; HP-HB) stars are the result of insufficient envelope masses to allow a full AGB phase, and are expected to be particularly prominent at high helium or α abundances when the mass loss on the RGB is high. Our data support previous claims that most UV-bright sources in the bulge are likely hot (extreme) horizontal branch stars (EHB) and their progeny. We construct the first radial profiles of these stellar populations, and show that they are highly centrally concentrated, even more so than the integrated UV or optical light. However, we find that this UV-bright population does not dominate the total UV luminosity at any radius, as we are detecting only the progeny of the EHB stars that are the likely source of the UVX. We calculate that only a few percent of MS stars in the central bulge can have gone through the HP-HB phase and that this percentage decreases strongly with distance from the center. We also find that the surface density of hot UV-bright stars has the same radial variation as that of low-mass X-ray binaries. We discuss age, metallicity, and abundance variations as possible explanations for the observed radial variation in the UV-bright population.

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“…Mass loss was applied in its most widely used formulation, namely the Reimers (1975Reimers ( , 1977 formula, with a range of values for the corresponding mass loss "efficiency parameter" η R that allowed us to produce from very red to very blue HB stars. While it is now known that the Reimers formulation does not properly describe mass loss in red giants (e.g., Schröder & Cuntz 2005;Catelan 2008, and references therein), the specific choice of mass loss formalism turns out to be unimportant for our present purposes.…”

Section: Evolutionary Tracks and Interpolation Gridmentioning

confidence: 99%

Stellar evolution and variability in the pre-ZAHB phase

Aguirre

Catelan

Weiß

et al. 2008

A&A

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Context. One of the most dramatic events in the life of a low-mass star is the He flash, which takes place at the tip of the red giant branch and is followed by a series of secondary flashes before the star settles on the zero-age horizontal branch (ZAHB). Yet, no stars have ever been positively identified in this key phase in the life of a low-mass star (hereafter the "pre-ZAHB" phase). Aims. In this paper, we investigate the possibility that at least some pre-ZAHB stars may cross the instability strip, thus becoming variable stars whose properties might eventually lead to positive identifications. In particular, it has been suggested that some of the RR Lyrae stars with high period change rates (Ṗ) may in fact be pre-ZAHB stars. Here we present the first theoretical effort devoted to interpreting at least some of the high-Ṗ stars as pre-ZAHB pulsators. Methods. We constructed an extensive grid of evolutionary tracks using the Garching Stellar Evolution Code (GARSTEC) for a chemical composition appropriate to the case of the globular cluster M 3 (NGC 5272), where a number of stars with highṖ values are found. We follow each star's pre-ZAHB evolution in detail, and compute the periods and period change rates for the stars lying inside the instability strip, also producing pre-ZAHB Monte Carlo simulations that are appropriate for the case of M 3. Results. Our results indicate that one should expect of order 1 pre-ZAHB star for every 60 or so bona fide HB stars in M 3. Among the pre-ZAHB stars, approximately 22% are expected to fall within the boundaries of the instability strip, presenting RR Lyrae-like pulsations. On average, these pre-ZAHB pulsators are expected to have longer periods than the bona fide HB pulsators, and 76% of them are predicted to show negativeṖ values. While the most likelyṖ value for the pre-ZAHB variables is ≈−0.3 d/Myr, more extremeṖ values are also possible: 38% of the variables are predicted to haveṖ < −0.8 d/Myr. Conclusions. It appears likely, therefore, that some -but certainly not all -of the RR Lyrae stars in M 3 with high (absolute)Ṗ values are in fact pre-ZAHB pulsators in disguise.

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Horizontal Branch Stars and the Ultraviolet Universe

Cited by 5 publications

References 80 publications

UV SEDs of early-type cluster galaxies: a new look at the UV upturn

UV SEDs of early-type cluster galaxies: a new look at the UV upturn

The Panchromatic Hubble Andromeda Treasury. I. Bright Uv Stars in the Bulge of M31

Stellar evolution and variability in the pre-ZAHB phase

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