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Post Info TOPIC: White Dwarfs
Blobrana


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RE: White Dwarfs
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Title: New mass limit of white dwarfs
Authors: Upasana Das, Banibrata Mukhopadhyay

Is the Chandrasekhar mass limit for white dwarfs (WDs) set in stone? Not anymore -- recent observations of over-luminous, peculiar type Ia supernovae can be explained if significantly super-Chandrasekhar WDs exist as their progenitors, thus barring them to be used as cosmic distance indicators. However, there is no estimate of a mass limit for these super-Chandrasekhar WD candidates yet. Can they be arbitrarily large? In fact, the answer is no! We arrive at this revelation by exploiting the flux freezing theorem in observed, accreting, magnetized WDs, which brings in Landau quantization of the underlying electron degenerate gas. This essay presents the calculations which pave the way for the ultimate (significantly super-Chandrasekhar) mass limit of WDs, heralding a paradigm shift 80 years after Chandrasekhar's discovery.

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Blobrana


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DA White Dwarfs
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Title: 72 DA White Dwarfs Identified in LAMOST Pilot Survey
Authors: J. K. Zhao, A. L. Luo, T.D. Oswalt, G. Zhao

We present a spectroscopically identified catalogue of 72 DA white dwarfs from the LAMOST pilot survey. 35 are found to be new identifications after cross-correlation with the Eisenstein et al. and Villanova catalogues. The effective temperature and gravity of these white dwarfs are estimated by Balmer lines fitting. Most of them are hot white dwarfs. The cooling times and masses of these white dwarfs are estimated by interpolation in theoretical evolution tracks. The peak of mass distribution is found to be ~ 0.6 solar masses which is consistent with prior work in the literature. The distances of these white dwarfs are estimated using the method of Synthetic Spectral Distances. All of these WDs are found to be in the Galactic disk from our analysis of space motions. Our sample supports the expectation white dwarfs with high mass are concentrated near the plane of Galactic disk

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Blobrana


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Posts: 131433
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RE: White Dwarfs
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Title: White dwarf cooling sequences and cosmochronology
Authors: J. Isern, A. Artigas, E. Garcia-Berro

The evolution of white dwarfs is a simple gravothermal process. This means that their luminosity function, i.e. the number of white dwarfs per unit bolometric magnitude and unit volume as a function of bolometric magnitude, is a monotonically increasing function that decreases abruptly as a consequence of the finite age of the Galaxy. The precision and the accuracy of the white dwarf luminosity functions obtained with the recent large surveys together with the improved quality of the theoretical models of evolution of white dwarfs allow to feed the hope that in a near future it will be possible to reconstruct the history of the different Galactic populations.

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Blobrana


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Title: Chandra grating spectroscopy of three hot white dwarfs
Authors: J. Adamczak, K. Werner, T. Rauch, S. Schuh, J. J. Drake, J. W. Kruk

High-resolution soft X-ray spectroscopic observations of single hot white dwarfs are scarce. With the Chandra Low-Energy Transmission Grating, we have observed two white dwarfs, one is of spectral type DA (LB 1919) and the other is a non-DA of spectral type PG1159 (PG 1520+525). The spectra of both stars are analysed, together with an archival Chandra spectrum of another DA white dwarf (GD 246). The soft X-ray spectra of the two DA white dwarfs are investigated in order to study the effect of gravitational settling and radiative levitation of metals in their photospheres. LB 1919 is of interest because it has a significantly lower metallicity than DAs with otherwise similar atmospheric parameters. GD 246 is the only white dwarf known that shows identifiable individual iron lines in the soft X-ray range. For the PG1159 star, a precise effective temperature determination is performed in order to confine the position of the blue edge of the GW Vir instability region in the HRD.

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Blobrana


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Title: The habitability and detection of Earth-like planets orbiting cool white dwarfs
Authors: L. Fossati, S. Bagnulo, C. A. Haswell, M. R. Patel, R. Busuttil, P. M. Kowalski, D. V. Shulyak, M. F. Sterzik

Since there are several ways planets can survive the giant phase of the host star, we examine the habitability and detection of planets orbiting white dwarfs. As a white dwarf cools from 6000 K to 4000 K, a planet orbiting at 0.01 AU would remain in the Continuous Habitable Zone (CHZ) for ~8 Gyr. We show that photosynthetic processes can be sustained on such planets. The DNA-weighted UV radiation dose for an Earth-like planet in the CHZ is less than the maxima encountered on Earth, hence non-magnetic white dwarfs are compatible with the persistence of complex life. Polarisation due to a terrestrial planet in the CHZ of a cool white dwarf is 10^2 (10^4) times larger than it would be in the habitable zone of a typical M-dwarf (Sun-like star). Polarimetry is thus a viable way to detect close-in rocky planets around white dwarfs. Multi-band polarimetry would also allow reveal the presence of a planet atmosphere, providing a first characterisation. Planets in the CHZ of a 0.6 M_sun white dwarf will be distorted by Roche geometry, and a Kepler-11d analogue would overfill its Roche lobe. With current facilities a Super-Earth-sized atmosphereless planet is detectable with polarimetry around the brightest known cool white dwarf. Planned future facilities render smaller planets detectable, in particular by increasing the instrumental sensitivity in the blue.

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Blobrana


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Title: Central stars of planetary nebulae: The white dwarf connection
Authors: K. Werner

This paper is focused on the transition phase between central stars and white dwarfs, i.e. objects in the effective temperature range 100,000 - 200,000 K. We confine our review to hydrogen-deficient stars because the common H-rich objects are subject of the paper by Ziegler et al. in these proceedings. We address the claimed iron-deficiency in PG1159 stars and [WC] central stars. The discovery of new Ne VII and Ne VIII lines in PG1159 stars suggests that the identification of O VII and O VIII lines that are used for spectral classification of [WCE] stars is wrong. We then present evidence for two distinct post-AGB evolutionary sequences for H-deficient stars based on abundance analyses of the He-dominated O(He) stars and the hot DO white dwarf KPD0005+5106. Finally, we report on evidence for an H-deficient post-super AGB evolution sequence represented by the hottest known, carbon/oxygen-atmosphere white dwarf H1504+65 and the recently discovered carbon-atmosphere "hot DQ" white dwarfs.

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Blobrana


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Title: A High Rate of White Dwarf-Neutron Star Mergers & Their Transients
Authors: Todd A. Thompson, Matthew D. Kistler, K. Z. Stanek

We argue that the recent groundbreaking discovery by Badenes et al. (2009) of a nearby (~50 pc) white dwarf-neutron star (or black hole) binary (SDSS 1257+5428) with a merger timescale ~500 Myr implies that such systems are common; we estimate that there are of order 10^6 in the Galaxy. Although subject to large uncertainties, the nominal derived merger rate is ~5 x 10^-4 per yr in the Milky Way, just ~3-6 and ~20-40 times less than the Type Ia and core-collapse supernova (SN) rates, respectively. This implies that the merger rate is ~0.5-1 x 10^4 per Gpc^3 per yr in the local universe, ~5000-10000 times more than the observed (beaming-uncorrected) long-duration gamma-ray burst (GRB) rate. We estimate the lower limit on the rate in the Galaxy to be >2.5 x 10^-5 per yr at 95% confidence. We briefly discuss the implications of this finding for the census of long- and short-duration GRBs and their progenitors, the frequency of tight binary companions to Type Ib/c SN progenitors, the origin of ultra-high energy cosmic rays (UHECRs), the formation of rapidly rotating neutron stars and ~2-3 M_sun black holes, the census of faint Ia-like SNe, as well as for upcoming and current transient surveys (e.g., LOSS, PTF, LSST), and for high- (LIGO) and low-frequency (LISA) gravitational wave searches.

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Blobrana


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Title: Two white dwarfs with oxygen-rich atmospheres
Authors: B.T. Gaensicke, D. Koester, J. Girven, T.R. Marsh, D. Steeghs

Stars with masses in the range 7-10Msun end their lives either as massive white dwarfs or weak type II supernovae, and there are only limited observational constraints of either channel. Here we report the detection of two white dwarfs with large photospheric oxygen abundances, implying that they are bare oxygen-neon cores and that they may have descended from the most massive progenitors that avoid core-collapse.

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Blobrana


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SDSS 0922+2928 and SDSS 1102+2054
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Astrophysicists at the University of Warwick and Kiel University have discovered two earth sized bodies with oxygen rich atmospheres -- however there is a bit of a disappointing snag for anyone looking for a potential home for alien life, or even a future home for ourselves, as they are not planets but are actually two unusual white dwarf stars.
The two white dwarf stars SDSS 0922+2928 and SDSS 1102+2054 are 400 and 220 light years from Earth. They are both the remnants of massive stars that are at the end of their stellar evolution having consumed all the material they had available for nuclear fusion.

SDSS11022054B.jpg
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SDSS1102+2054
Credit: The Sloan Digital Sky Survey

Theoretical models suggest that massive stars (around 7 - 10 times the mass of our own Sun) will consume all of their hydrogen, helium and carbon, and end their lives either as white dwarfs with very oxygen-rich cores, or undergo a supernova and collapse into neutron stars. Finding such oxygen-rich white dwarfs would be an important confirmation of the models.

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Blobrana


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Posts: 131433
Date:
RE: White Dwarfs
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Pulsating White Dwarfs explained by Victorias "accidental astronomer"
Exploring distant parts of the galaxy, astrophysicist Denis Sullivan has collaborated on the discovery of about six extrasolar planets - not bad for an accidental astronomer.
Next Tuesday Professor Sullivan will discuss his most recent research into some of the universe's deepest mysteries in his inaugural professorial lecture at Victoria University of Wellington.

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