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G. Fontaine

Publications and source records attributed to G. Fontaine.

At least 235 records · Page 13Linked to original sources
arXiv2005

Towards an Empirical Determination of the ZZ Ceti Instability Strip

We present atmospheric parameters for a large sample of DA white dwarfs that are known to be photometrically constant. For each star, we determine the effective temperature and surface gravity by comparing high signal-to-noise ratio optical spectra to the predictions of detailed model atmosphere calculations. We also report the successful prediction and detection of photometric variability in G232-38 based on similar Teff and log g determinations. The atmospheric parameters derived for this sample of constant stars as well as those for the known sample of bright ZZ Ceti stars (now boosted to a total of 39) have been obtained in a highly homogeneous way. We combine them to study the empirical red and blue edges as well as the purity of the ZZ Ceti instability strip. We find that the red edge is rather well constrained whereas there exists a rather large range of possibilities for the slope of the blue edge. Furthermore, the ZZ Ceti instability strip that results from our analysis contains no nonvariable white dwarfs. Our sample of constant stars is part of a much broader spectroscopic survey of bright (V < 17) DA white dwarfs, which we have recently undertaken. We also present here some preliminary results of this survey. Finally, we revisit the analysis by Mukadam et al. of the variable and nonvariable DA stars uncovered as part of the Sloan Digital Sky Survey. Their erroneous conclusion of an instability strip containing several nonvariable stars is traced back to the low signal-to-noise ratio spectroscopic observations used in that survey.

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arXiv2005

The Nature of the Driving Mechanism in the Pulsating Hybrid PG 1159 Star Abell 43

We extend our previous pulsational stability analyses of PG 1159 stars by modeling the hybrid PG 1159 type star Abell 43. We show that the standard kappa-mechanism due to the ionization of C and O in the envelope of this H-rich PG 1159 star is perfectly able to drive g-mode pulsations. Thus, contrary to a recent suggestion, there is no need to invoke any new or exotic mechanism to explain the pulsational instabilities observed in this particular star. Our expected instability band for l = 1 modes extends in period from ~ 2604 s to ~ 5529 s, which is consistent with the available photometric observations of Abell 43. We also suggest that efforts to detect luminosity variations in its sibling NGC 7094 be pursued.

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arXiv2005

A Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

We present an analysis of spectroscopic and photometric observations of cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron, Leggett, & Ruiz, and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey. Our analysis confirms that effective temperatures derived from model atmospheres including carbon are significantly lower than the temperatures obtained from pure helium models. Similarly the mean mass of the trigonometric parallax sample, = 0.62 Mo, is significantly lower than that obtained from pure helium models, = 0.73 Mo, and more consistent with the spectroscopic mean mass of DB stars, = 0.59 Mo, the most likely progenitors of DQ white dwarfs. We find that DQ stars form a remarkably well defined sequence in a carbon abundance versus effective temperature diagram; below Teff~10,000 K, carbon pollution decreases monotonically with decreasing effective temperature. Improved evolutionary models including diffusion and connecting to the PG 1159 phase are used to infer a typical value for the thickness of the helium layer M_He/M_* between 10^{-3} and 10^{-2}, compatible with the predictions of post-AGB models.

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arXiv2005

Structural Parameters of the Hot Pulsating B Subdwarf PG 1219+534 from Asteroseismology

We have carried out a detailed asteroseismic analysis of PG 1219+534. Our analysis leads to the identification of the (k,l) indices of nine periods observed in the star PG 1219+534, and to the determination of its structural parameters. The periods all correspond to low-order acoustic modes with adjacent values of k and with l=0, 1, 2, and 3. They define a band of unstable modes, in close agreement with nonadiabatic pulsation theory. Furthermore, the average dispersion between the nine observed periods and the periods of the corresponding nine theoretical modes of the optimal model is only ~0.6%, comparable to the results of a similar analysis carried out by Brassard et al. (2001) on the rapid sdB pulsator PG 0014+067. On the basis of our combined spectroscopic and asteroseismic analysis, the inferred global structural parameters of PG 1219+534 are Teff = 33,600 +/- 370 K, log g = 5.8071 +/- 0.0057, log Menv/M* = -4.254 +/- 0.147, M* = 0.457 +/- 0.012 Msun, R/Rsun = 0.1397 +/- 0.0028, and L/Lsun = 22.01 +/- 1.85. Combined with detailed model atmosphere calculations, we estimate, in addition, that this star has an absolute visual magnitude MV = 4.62 +/- 0.06 and is located at a distance d = 531 +/- 23 pc (using V = 13.24 +/- 0.20). Finally, if we interpret the absence of fine structure (frequency multiplets) as indicative of a slow rotation rate of that star, we further find that PG 1219+534 rotates with a period longer than 3.4 days, and has a maximum rotational broadening velocity of Vsin i <~ 2.1 km/s.

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arXiv2005

Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

We present an analysis of spectroscopic and photometric data for cool DQ white dwarfs based on improved model atmosphere calculations. In particular, we revise the atmospheric parameters of the trigonometric parallax sample of Bergeron et al.(2001), and discuss the astrophysical implications on the temperature scale and mean mass, as well as the chemical evolution of these stars. We also analyze 40 new DQ stars discovered in the first data release of the Sloan Digital Sky Survey.

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arXiv2004

Discovery of a Long-Period Photometric Variation in the V361 Hya Star HS 0702+6043

We report the discovery of a long-period g-mode oscillation in the previously known short-period p-mode sdB pulsator HS 0702+6043. This makes this star an extraordinary object, unique as a member of the family of sdB pulsators, and one of the very few known pulsating stars overall exhibiting excited modes along both the acoustic and gravity branches of the nonradial pulsation spectrum. Because p-modes and g-modes probe different regions of a pulsating star, HS 0702+6043 holds a tremendous potential for future detailed asteroseismological investigations.

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arXiv2004

White Dwarfs and Hot Subdwarfs as Seen from FUSE

We present a small collection of FUSE spectra representative of the main spectral classes found in white dwarf stars. In addition, we also discuss another family of hot evolved stars, that of the hot subdwarfs. Both families belong to the chemically peculiar stars, and it is thought that a complex interplay of competing processes such as gravitational settling, ordinary diffusion, radiative levitation, weak stellar winds, and accretion is responsible for the rich variety of atmospheric compositions observed in those objects. FUSE is playing a key role in the current quest for establishing a coherent theory of the spectral evolution of these stars as it allows the determination of the patterns of heavy element abundances at a significantly higher level of accuracy than has been possible before on the basis of optical or UV observations. We also briefly present some fascinating FUV light curves of a handful of pulsating subdwarf B stars, thus illustrating the unique potential of FUSE for asteroseismological studies, a potential which has not been exploited yet.

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arXiv2003

On the Purity of the ZZ Ceti Instability Strip: Discovery of More Pulsating DA White Dwarfs on the Basis of Optical Spectroscopy

We report the discovery of two new ZZ Ceti pulsators, LP 133-144 and HE 1258+0123, selected on the basis of model atmosphere fits to optical spectroscopic data. The atmospheric parameters for LP 133-144, Teff = 11,800 +/- 200 K and log g = 7.87 +/- 0.05, and for HE 1258+0123, Teff = 11,410 +/- 200 K and log g = 8.04 +/- 0.05, place them within the empirical boundaries of the ZZ Ceti instability strip. This brings the number of known ZZ Ceti stars to a total of 36, a quarter of which have now been discovered using the spectroscopic approach for estimating their atmospheric parameters. This method has had a 100% success rate so far in predicting the variability of candidate ZZ Ceti stars. We have also analyzed additional spectra of known nonvariable white dwarfs in the vicinity of the ZZ Ceti instability strip. Our study further strengthens the idea that ZZ Ceti stars occupy a pure region in the log g-Teff plane, a region where no nonvariable stars are found. This result supports the thesis that ZZ Ceti pulsators represent a phase through which all DA stars must evolve.

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arXiv2003

HST/STIS observations of sdBV stars: testing diffusion and pulsation theory

We present the initial results of an abundance analysis of echelle UV spectra of 5 hot subdwarf B (sdB) stars. These stars have been identified as core helium burning objects on the extreme Horizontal Branch. Around 5% of sdBs show short-period acoustic mode oscillations. Models predict that these oscillations are due to an opacity bump caused by the ionisation of iron group elements. The necessary metal abundance has to be maintained by diffusive equilibrium between gravitational settling and radiative levitation. However, analyses of high resolution optical spectra has revealed that we cannot discriminate between pulsating and non-pulsating sdBs on the basis of the surface iron abundance. We have therefore obtained HST/STIS observations of 3 pulsators and 2 non-pulsators in the near and far UV to measure the surface abundance of elements that are unobservable from the ground. The overall aim of our study is to test diffusion and pulsation calculations by searching for significant differences between these surface abundances.

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arXiv2002

Discovery of A New Class of Pulsating Stars: Gravity-mode Pulsators among Subdwarf B Stars

During the course of an ongoing CCD monitoring program to investigate low-level light variations in subdwarf B (sdB) stars, we have serendipitously discovered a new class of low amplitude, multimode sdB pulsators with periods of the order of an hour. These periods are more than a factor of ten longer than those of previously known multimode sdB pulsators (EC 14026 stars), implying that they are due to gravity modes rather than pressure modes. The longer period pulsators are found only among cooler sdB stars, where they are surprisingly common. The iron opacity instability that drives the short period EC 14026 stars is effective only in hot sdB's, leaving the driving mechanism for the deeper gravity modes in cool sdB's currently unknown. We present the first observational results for our newly identified sdB variables, and discuss possible implications.

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arXiv2002

Detection of the BL Lac object 1ES1426+428 in the Very High Energy gamma-ray band by the CAT Telescope from 1998-2000

The BL Lac Object 1ES 1426+428, at a red-shift of z=0.129, has been monitored by the CAT telescope from February 1998 to June 2000. The accumulation of 26 hours of observations shows a gamma-ray signal of 321 events above 250 GeV at 5.2 standard deviations, determined using data analysis cuts adapted to a weak, steep-spectrum source. The source emission has an average flux of Phi_diff(400 GeV) = 6.73 +/- 1.27stat +/- 1.45syst x 10^-11 /cm^-2/s/TeV, and a very steep spectrum, with a differential spectral index of gamma = -3.60 +/- 0.57 which can be refined to gamma = -3.66 +/- 0.41 using a higher flux data subset. If, as expected from its broad-band properties, the Very High Energy emission is hard at the source, these observations support a strong absorption effect of gamma-rays by the Intergalactic Infrared field.

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arXiv2001

Temporal and spectral gamma-ray properties of Mkn421 above 250 GeV from CAT observations between 1996 and 2000

The gamma-ray emission above 250 GeV from the BL Lac object Markarian 421 was observed by the CAT Cherenkov imaging telescope between December, 1996, and June, 2000. In 1998, the source produced a series of small flares, making it the second extragalactic source detected by CAT. The time-averaged differential spectrum has been measured from 0.3 to 5 TeV, which is well fitted with a power law with an index of -2.88+-0.12(stat)+-0.06(syst). In 2000, the source showed an unprecedented activity, with variability time-scales as short as one hour, as for instance observed during the night between 4 and 5 February. The 2000 time-averaged spectrum measured is compatible with that of 1998, but some indication of a spectral curvature is found between 0.3 and 5 TeV. The possibility of TeV spectral hardening during flares is also discussed, and the results are compared to those obtained on the other TeV BL Lac, Markarian 501.

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arXiv2000

Very High Energy Gamma-ray spectral properties of Mrk 501 from CAT Cerenkov telescope observations in 1997

The BL Lac object Mrk 501 went into a very high state of activity during 1997, both in VHE gamma-rays and X-rays. We present here results from observations at energies above 250 GeV carried out between March and October 1997 with the CAT Cerenkov imaging Telescope. The average differential spectrum between 30 GeV and 13 TeV shows significant curvature and is well represented by phi_0 * E_TeV^{-(alpha + beta*log10(E_TeV))}, with: phi_0 = 5.19 +/- 0.13 {stat} +/- 0.12 {sys-MC} +1.66/-1.04 {sys-atm} * 10^-11 /cm^2/s/TeV alpha = 2.24 +/- 0.04 {stat} +/- 0.05 {sys} beta = 0.50 +/- 0.07 {stat} (negligible systematics). The TeV spectral energy distribution of Mrk 501 clearly peaks in the range 500 GeV-1 TeV. Investigation of spectral variations shows a significant hardness-intensity correlation with no measurable effect on the curvature. This can be described as an increase of the peak TeV emission energy with intensity. Simultaneous and quasi-simultaneous CAT VHE gamma-ray and BeppoSAX hard X-ray detections for the highest recorded flare on 16th April and for lower-activity states of the same period show correlated variability with a higher luminosity in X-rays than in gamma-rays. The observed spectral energy distribution and the correlated variability between X-rays and gamma-rays, both in amplitude and in hardening of spectra, favour a two-component emission scheme where the low and high energy components are attributed to synchrotron and inverse Compton (IC) radiation, respectively.

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arXiv1998

A new analysis method for very high definition Imaging Atmospheric Cherenkov Telescopes as applied to the CAT telescope

A new method of shower-image analysis is presented which appears very powerful as applied to those Cherenkov Imaging Telescopes with very high definition imaging capability. It provides hadron rejection on the basis of a single cut on the image shape, and simultaneously determines the energy of the electromagnetic shower and the position of the shower axis with respect to the detector. The source location is also reconstructed for each individual gamma-ray shower, even with one single telescope, so for a point source the hadron rejection can be further improved. As an example, this new method is applied to data from the CAT (Cherenkov Array at Themis) imaging telescope, which has been operational since Autumn, 1996.

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arXiv1998

The CAT Imaging Telescope for Very-High-Energy Gamma-Ray Astronomy

The CAT (Cherenkov Array at Themis) imaging telescope, equipped with a very-high-definition camera (546 fast phototubes with 0.12 degrees spacing surrounded by 54 larger tubes in two guard rings) started operation in Autumn 1996 on the site of the former solar plant Themis (France). Using the atmospheric Cherenkov technique, it detects and identifies very high energy gamma-rays in the range 250 GeV to a few tens of TeV. The instrument, which has detected three sources (Crab nebula, Mrk 421 and Mrk 501), is described in detail.

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arXiv1997

Understanding the Cool DA White Dwarf, G29-38

The white dwarfs are promising laboratories for the study of cosmochronology and stellar evolution. Through observations of the pulsating white dwarfs, we can measure their internal structures and compositions, critical to understanding post main sequence evolution, along with their cooling rates, allowing us to calibrate their ages directly. The most important set of white dwarf variables to measure are the oldest of the pulsators, the cool DAVs, which have not previously been explored through asteroseismology due to their complexity and instability. Through a time-series photometry data set spanning ten years, we explore the pulsation spectrum of the cool DAV, G29-38 and find an underlying structure of 19 (not including multiplet components) normal-mode, probably l=1 pulsations amidst an abundance of time variability and linear combination modes. Modelling results are incomplete, but we suggest possible starting directions and discuss probable values for the stellar mass and hydrogen layer size. For the first time, we have made sense out of the complicated power spectra of a large-amplitude DA pulsator. We have shown its seemingly erratic set of observed frequencies can be understood in terms of a recurring set of normal-mode pulsations and their linear combinations. With this result, we have opened the interior secrets of the DAVs to future asteroseismological modelling, thereby joining the rest of the known white dwarf pulsators.

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arXiv1997

Performances of the CAT imaging telescope and some preliminary results on Mkn 180 and the Crab

The CAT imaging telesope, which uses the Atmospheric Cherenkov Technique to observe TeV gamma-ray sources, has been operating since September 1996. Located in southern France, it features a fine grained camera consisting of 546 PMT with a pixel size of 0.12 degrees. Strong gamma-ray signals from Mkn 501 above 200 GeV were detected during observations made in April 1997. The high signal/noise ratio for these data enables direct characterization of the gamma-ray images as recorded in the camera. These results are used to validate the Monte-Carlo simulations of the response of the telescope to gamma-ray showers and to improve its performances. Observational results on Mkn 180 and the Crab nebula spectrum are presented.

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