UV Upturn In Elliptical Galaxies Research Articles

HOW TO MAKE A SINGLETON sdB STAR VIA ACCELERATED STELLAR EVOLUTION

Many hot subdwarf B stars (sdBs) are in close binaries, and the favored formation channels for subdwarfs rely on mass transfer in a binary system to strip a core He burning star of its envelope. However, these channels cannot account for sdBs that have been observed in long period binaries nor the narrow mass distribution of isolated (or "singleton") sdBs. We propose a new formation channel involving the merger of a helium white dwarf and a low mass, hydrogen burning star, which addresses these issues. Hierarchical triples whose inner binaries merge and form sdBs by this process could explain the observed long period subdwarf+main sequence binaries. This process would also naturally explain the observed slow rotational speeds of singleton sdBs. We also briefly discuss the implications of this formation channel for extreme horizontal branch morphology in globular clusters and the UV upturn in elliptical galaxies.

Influence of α-enhancement on effective temperatures of hot subdwarfs

Observations of metal elements in some halo or bulge globular clusters and in some elliptical galaxies have shown the α-enhancement phenomenon. Hot subdwarfs are important stars, e.g., they are important sources of far-ultraviolet light in the Galaxy and have successfully been used to explain the UV-upturn in elliptical galaxies. Therefore, we try to study the impacts of α-enhancement on hot subdwarfs. In our calculation, the value of the metallicity is 0.02, all the core masses of hot subdwarfs are 0.475 M ⊙, and the envelope masses of the hot subdwarfs are 0.001, 0.005 and 0.016 M ⊙, respectively. We find that the influences of α-enhancement on luminosity and evolutionary age of hot subdwarfs are not visible. But α-enhancement can make the effective temperatures of hot subdwarfs become higher, and the thicker their envelopes are, the more obvious this influence is.

UV Upturn in Elliptical Galaxies: Theory

The UV upturn phenomenon found in bright elliptical galaxies has notable implications for stellar evolution, galaxy evolution and even for cosmology. The recent space observations of three galaxy clusters made by Brown and his collaborators shed great lights on this field of study. Theoretical models are being improved based on such data. While the core-helium burning stars remain the strongest candidate for the UV upturn, there is a growing concern that the canonical models may be missing important elements, such as binaries. I review the current theoretical understanding and future prospects.

The UV Upturn in Elliptical Galaxies

The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest "UV upturns," while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al., 1994), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon.

On the Origin of the UV Upturn in Elliptical Galaxies. I. Sensitivity of UV Population Synthesis to Various Input Parameters

We present models of the late stages of stellar evolution intended to explain the UV upturn phenomenon in elliptical galaxies. Such models are sensitive to values of a number of poorly-constrained physical parameters, including metallicity, age, stellar mass loss, helium enrichment, and the distribution of stars on the zero age horizontal branch (HB). We explore the sensitivity of the results to values of these parameters, and reach the following conclusions. Old, metal rich galaxies, such as giant ellipticals, naturally develop a UV upturn within a reasonable time scale - less than a Hubble time - without the presence of young stars. The most likely stars to dominate the UV flux of such populations are low mass, core helium burning (HB and evolved HB) stars. Metal-poor populations produce a higher ratio of UV-to-V flux, due to opacity effects, but only metal-rich stars develop a UV upturn, in which the flux increases towards shorter UV wavelengths. Model color-magnitude diagrams and corresponding integrated spectra (for various values of age, metallicity, helium enrichment, mass loss efficiency, initial mass function, and the HB mass dispersion factor) are available on S.Y.'s world wide web site http://shemesh.gsfc.nasa.gov/model.html.

The UV Upturn in Elliptical Galaxies

The hot stellar component in elliptical galaxies offers clues to both stellar evolution and galaxy evolution. Current observations suggest that extreme horizontal branch (EHB) stars dominate the far-UV emission from galaxies with the strongest “UV upturns,” while post asymptotic giant branch (PAGB) stars are probably significant contributors for weaker galaxies. Spectra near the Lyman limit indicate that a rather narrow range of temperature (and hence EHB star mass) is required. However, other arguments suggest that most of the helium-burning stars in elliptical galaxies are in the red clump. The HB star mass distribution therefore appears to be strongly bimodal. Such bimodality is qualitatively reproduced by two radically different stellar population models, (those of Lee and Bressan et al.), both of which require that the galaxies be very old. However, the Galactic open cluster NGC 6791 also contains EHB stars and exhibits strong bimodality, indicating that old age may not necessarily be a requirement for the UV upturn phenomenon.

Constraints on the origin of the ultraviolet upturn in elliptical galaxies from Hopkins Ultraviolet Telescope observations of NGC 1399

view Abstract Citations (100) References (39) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Constraints on the Origin of the Ultraviolet Upturn in Elliptical Galaxies from Hopkins Ultraviolet Telescope Observations of NGC 1399 Ferguson, Henry C. ; Davidsen, Arthur F. ; Kriss, Gerard A. ; Blair, William P. ; Bowers, Charles W. ; Dixon, W. Van Dyke ; Durrance, Samuel T. ; Feldman, Paul D. ; Henry, Richard C. ; Kruk, Jeffrey W. ; Moos, H. Warren ; Vancura, Olaf ; Long, Knox S. ; Kimble, Randy A. Abstract The giant elliptical galaxy NGC 1399 was observed with the Hopkins Ultraviolet Telescope (HUT) aboard the Astro-1 space shuttle mission in 1990 December. These are the first data to provide measurements of the spectral energy distribution of an elliptical galaxy shortward of Lyα and to allow detection of absorption features in the2000 A UV. The spectrum shows no emission features other than well-known geocoronal lines. Strong Lyman series absorption is evident at the galaxy redshift. Other absorption features attributable to the hot stellar population are detected, but there is no evidence of C IV λλ1548, 1551 absorption. We use these data to set constraints on the origin of the UV upturn in elliptical galaxies. The lack of detectable C IV absorption and the shape of the continuum spectrum exclude star formation with a standard IMF as the sole contributor to the 1550 A flux. The spectral energy distributions predicted by the "hot-E" model of Guiderdoni and Rocca- Volmerange and the Bruzual μ-models do not match the observed spectrum. Post-asymptotic giant branch (PAGB) stars can account for the spectrum near 1550 A, because many have low carbon abundance in their envelopes. However, synthetic spectra produced from theoretical evolutionary tracks and stellar model atmospheres fail to fit to the observed spectral-energy distribution. If PAGB stars produce the UV upturn, then either they do not evolve along the standard tracks, or the (solar metallicity) model atmospheres used to construct the synthetic spectrum are inappropriate. The observed continuum flux decreases from 1050 A to the Lyman limit, indicating that the light is dominated by stars with temperatures less than 25,000 K. For star-formation models, this temperature puts the main-sequence turnoff near BO, implying either that star formation ended ~2 x 10^7^ yr ago, or that stars are currently forming with a truncated IMF. The continuum shape and presence of Lyman- series absorption suggest extreme horizontal-branch star models deserve additional attention. Publication: The Astrophysical Journal Pub Date: December 1991 DOI: 10.1086/186215 Bibcode: 1991ApJ...382L..69F Keywords: Elliptical Galaxies; Galactic Evolution; Spaceborne Astronomy; Ultraviolet Astronomy; Ultraviolet Spectra; Astro Missions (Sts); Asymptotic Giant Branch Stars; Main Sequence Stars; Spectral Energy Distribution; Stellar Evolution; Ultraviolet Telescopes; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 1399; GALAXIES: STELLAR CONTENT; ULTRAVIOLET: SPECTRA full text sources ADS | data products MAST (2) SIMBAD (1) NED (1)