60 research outputs found
Laboratory Measurements Of White Dwarf Photospheric Spectral Lines: H Beta
We spectroscopically measure multiple hydrogen Balmer line profiles from laboratory plasmas to investigate the theoretical line profiles used in white dwarf (WD) atmosphere models. X-ray radiation produced at the Z Pulsed Power Facility at Sandia National Laboratories initiates plasma formation in a hydrogen-filled gas cell, replicating WD photospheric conditions. Here we present time-resolved measurements of H beta and fit this line using different theoretical line profiles to diagnose electron density, n(e), and n = 2 level population, n2. Aided by synthetic tests, we characterize the validity of our diagnostic method for this experimental platform. During a single experiment, we infer a continuous range of electron densities increasing from n(e) similar to 4 to similar to 30 x 10(16) cm(-3) throughout a 120-ns evolution of our plasma. Also, we observe n(2) to be initially elevated with respect to local thermodynamic equilibrium (LTE); it then equilibrates within similar to 55 ns to become consistent with LTE. This supports our electrontemperature determination of T-e similar to 1.3 eV (similar to 15,000 K) after this time. At n(e) greater than or similar to 10(17) cm(-3), we find that computer-simulation-based line-profile calculations provide better fits (lower reduced chi(2)) than the line profiles currently used in the WD astronomy community. The inferred conditions, however, are in good quantitative agreement. This work establishes an experimental foundation for the future investigation of relative shapes and strengths between different hydrogen Balmer lines.Laboratory Directed Research and Development programUnited States Department of Energy DE-AC04-94AL85000, DE-SC0010623National Science Foundation DGE-1110007Astronom
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DBA and DB Stars
We measure apparent velocities (v_app) of absorption lines for 36 white
dwarfs (WDs) with helium-dominated atmospheres -- 16 DBAs and 20 DBs -- using
optical spectra taken for the European Southern Observatory SN Ia progenitor
survey (SPY). We find a difference of 6.9+/-6.9 km/s in the average apparent
velocity of the H-alpha lines versus that of the HeI 5876AA for our DBAs. This
is a measure of the blueshift of this He line due to pressure effects. By using
this as a correction, we extend the gravitational redshift method employed by
Falcon et al. (2010) to use the apparent velocity of the HeI 5876AA line and
conduct the first gravitational redshift investigation of a group of WDs
without visible hydrogen lines. We use biweight estimators to find an average
apparent velocity, _BI, (and hence average gravitational redshift,
_BI) for our WDs; from that we derive an average mass, _BI. For the
DBAs, we find _BI = 40.8+/-4.7 km/s and derive _BI = 0.71 +0.04 -0.05
Msun. Though different from of DAs (32.57 km/s) at the 91% confidence
level and suggestive of a larger DBA mean mass than that for normal DAs derived
using the same method (0.647 +0.013 -0.014 Msun; Falcon et al. 2010), we do not
claim this as a stringent detection. Rather, we emphasize that the difference
between _BI of the DBAs and of normal DAs is no larger than 9.2
km/s, at the 95% confidence level; this corresponds to roughly 0.10 Msun. For
the DBs, we find ^He_BI = 42.9+/-8.49 km/s after applying the blueshift
correction and determine _BI = 0.74 +0.08 -0.09 Msun. The difference between
^He_BI of the DBs and of DAs is less than or equal to 11.5 km/s
(~0.12 Msun), at the 95% confidence level. The gravitational redshift method
indicates much larger mean masses than the spectroscopic determinations of the
same sample by Voss et al. (2007)...Comment: Accepted to the Astrophysical Journal, 10 pages double-column, 3
figures, 5 table
A Gravitational Redshift Determination of the Mean Mass of White Dwarfs. DA Stars
We measure apparent velocities (v_app) of the Halpha and Hbeta Balmer line
cores for 449 non-binary thin disk normal DA white dwarfs (WDs) using optical
spectra taken for the ESO SN Ia Progenitor surveY (SPY; Napiwotzki et al.
2001). Assuming these WDs are nearby and co-moving, we correct our velocities
to the Local Standard of Rest so that the remaining stellar motions are random.
By averaging over the sample, we are left with the mean gravitational redshift,
: we find = = 32.57 +/- 1.17 km/s. Using the mass-radius
relation from evolutionary models, this translates to a mean mass of 0.647
+0.013 -0.014 Msun. We interpret this as the mean mass for all DAs. Our results
are in agreement with previous gravitational redshift studies but are
significantly higher than all previous spectroscopic determinations except the
recent findings of Tremblay & Bergeron (2009). Since the gravitational redshift
method is independent of surface gravity from atmosphere models, we investigate
the mean mass of DAs with spectroscopic Teff both above and below 12000 K; fits
to line profiles give a rapid increase in the mean mass with decreasing Teff.
Our results are consistent with no significant change in mean mass: ^hot =
0.640 +/- 0.014 Msun and ^cool = 0.686 +0.035 -0.039 Msun.Comment: Accepted for publication in ApJ, 14 pages, 12 figure
Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti
We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom
Measuring The Evolutionary Rate Of Cooling Of ZZ Ceti
We have finally measured the evolutionary rate of cooling of the pulsating hydrogen atmosphere (DA) white dwarf ZZ Ceti (Ross 548), as reflected by the drift rate of the 213.13260694 s period. Using 41 yr of time-series photometry from 1970 November to 2012 January, we determine the rate of change of this period with time to be dP/dt = (5.2 +/- 1.4) x 10(-15) s s(-1) employing the O - C method and (5.45 +/- 0.79) x 10(-15) s s(-1) using a direct nonlinear least squares fit to the entire lightcurve. We adopt the dP/dt obtained from the nonlinear least squares program as our final determination, but augment the corresponding uncertainty to a more realistic value, ultimately arriving at the measurement of dP/dt = (5.5 +/- 1.0) x 10(-15) s s(-1). After correcting for proper motion, the evolutionary rate of cooling of ZZ Ceti is computed to be (3.3 +/- 1.1) x 10(-15) s s(-1). This value is consistent within uncertainties with the measurement of (4.19 +/- 0.73) x 10(-15) s s(-1) for another similar pulsating DA white dwarf, G 117-B15A. Measuring the cooling rate of ZZ Ceti helps us refine our stellar structure and evolutionary models, as cooling depends mainly on the core composition and stellar mass. Calibrating white dwarf cooling curves with this measurement will reduce the theoretical uncertainties involved in white dwarf cosmochronometry. Should the 213.13 s period be trapped in the hydrogen envelope, then our determination of its drift rate compared to the expected evolutionary rate suggests an additional source of stellar cooling. Attributing the excess cooling to the emission of axions imposes a constraint on the mass of the hypothetical axion particle.NSF AST-1008734, AST-0909107Norman Hackerman Advanced Research Program 003658-0252-2009Astronom
Photometric variability in a warm, strongly magnetic DQ white dwarf, SDSS J103655.39+652252.2
We present the discovery of photometric variability in the DQ white dwarf
SDSS J103655.39+652252.2 (SDSS J1036+6522). Time-series photometry reveals a
coherent monoperiodic modulation at a period of 1115.64751(67) s with an
amplitude of 0.442% +/- 0.024%; no other periodic modulations are observed with
amplitudes >~0.13%. The period, amplitude, and phase of this modulation are
constant within errors over 16 months. The spectrum of SDSS J1036+6522 shows
magnetic splitting of carbon lines, and we use Paschen-Back formalism to
develop a grid of model atmospheres for mixed carbon and helium atmospheres.
Our models, while reliant on several simplistic assumptions, nevertheless match
the major spectral and photometric properties of the star with a
self-consistent set of parameters: Teff~15,500 K, log g ~9, log(C/He)=-1.0, and
a mean magnetic field strength of 3.0 +/- 0.2 MG. The temperature and
abundances strongly suggest that SDSS J1036+6522 is a transition object between
the hot, carbon-dominated DQs and the cool, He-dominated DQs. The variability
of SDSS J1036+6522 has characteristics similar to those of the variable hot
carbon-atmosphere white dwarfs (DQVs), however, its temperature is
significantly cooler. The pulse profile of SDSS J1036+6522 is nearly
sinusoidal, in contrast with the significantly asymmetric pulse shapes of the
known magnetic DQVs. If the variability in SDSS J1036+6522 is due to the same
mechanism as other DQVs, then the pulse shape is not a definitive diagnostic on
the absence of a strong magnetic field in DQVs. It remains unclear whether the
root cause of the variability in SDSS J1036+6522 and the other hot DQVs is the
same.Comment: Accepted for publication in ApJ. 12 pages, 9 figure
The Berkeley sample of Type II supernovae: BVRI light curves and spectroscopy of 55 SNe II
In this work, BVRI light curves of 55 Type II supernovae (SNe II) from the Lick Observatory Supernova Search programme obtained with the Katzman Automatic Imaging Telescope and the 1 m Nickel telescope from 2006 to 2018 are presented. Additionally, more than 150 spectra gathered with the 3 m Shane telescope are published. We conduct an analyse of the peak absolute magnitudes, decline rates, and time durations of different phases of the light and colour curves. Typically, our light curves are sampled with a median cadence of 5.5 d for a total of 5093 photometric points. In average, V-band plateau declines with a rate of 1.29 mag (100 d)−1, which is consistent with previously published samples. For each band, the plateau slope correlates with the plateau length and the absolute peak magnitude: SNe II with steeper decline have shorter plateau duration and are brighter. A time-evolution analysis of spectral lines in term of velocities and pseudo-equivalent widths is also presented in this paper. Our spectroscopic sample ranges between 1 and 200 d post-explosion and has a median ejecta expansion velocity at 50 d post-explosion of 6500 km s−1 (H α line) and a standard dispersion of 2000 km s−1. Nebular spectra are in good agreement with theoretical models using a progenitor star having a mass <16M⊙. All the data are available to the community and will help to understand SN II diversity better, and therefore to improve their utility as cosmological distance indicators
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