Search

We present a preliminary analysis of the recently discovered carbon‐dominated atmosphere white dwarfs (“Hot DQ”) based on HST/COS observations. We also present a summary of the recent findings that emerged from our ongoing campaign aimed at obtaining high signal‐to‐noise optical spectroscopic observations of these objects.

The three papers presented in this section on model atmospheres of brown dwarfs raise several common themes. I discuss three of these from an observational viewpoint — the temperature scale for L dwarfs, the role of the alkali elements, and the liely “weather” in brown dwarf atmospheres.

The session was brief and quite informal as there were only six participants. The agenda included my report on organizational activities of the Commission during the 2003-2006 term and Virginia Trimble's presentation Presence of binary stars in the current astronomical literature. I summarize below the most important part of my report.

Precise trigonometric parallax measurements orders of magnitude more accurate than a milliarcsecond will contribute greatly to our understanding of peculiar, low luminosity stellar objects of several types. First, the volume of space out to which luminosities may be determined to the accuracy of the best, very-nearby stars will be increased greatly. For the relatively rare field Population II stars, this will lead to the first accurate empirical calibrations of the main sequence at the low mass end, for comparison with globular clusters of various metallicities. Parallaxes at 1 kpc or farther will be adequate to help in the discovery or confirmation of the rare carbon dwarfs — main sequence stars with carbon-rich atmospheres. For cool white dwarfs, luminosities accurate to a few per cent or better will identify unresolved binaries, and objects of unusually high and low mass. For our most numerous solar neighbors, the M dwarfs and especially those near the stellar mass limit, accurate luminosities can help in the determinations of the chemical composition and age distributions.

We present initial results of an all-sky search for cool, low-mass stars and brown dwarfs within 20 pc. We have used the near-infrared photometry provided by the 2-Micron All Sky Survey (2MASS) Second Incremental Data Release to create a target list of 1793 candidates with 6 < J < 17 and J – K > 1. Follow-up observations and existing data have enabled over 85% of the sample to be classified. Spectral types and distances have been derived for over 250 nearby dwarfs, including 39 newly discovered L dwarfs. In this poster, we discuss our selection methods and present preliminary results.

Results are presented from an X-ray survey of ∼50 square degrees of the high galactic latitude sky at sensitivities in the range 7·10−14–5·10−12 erg/cm2 sec (0.3–3.5 keV) carried out with the Imaging Proportional Counter (IPC) aboard the Einstein Observatory. The extragalactic sample consists of 48 sources which have been used to determine the number flux relation. The content of the sample is analyzed in terms of types of sources and is found to be significantly different from the content of similar samples selected at higher fluxes.

Observations a haute dispersions de la region spectrale Hα pour un echantillon d'etoiles naines M tres avancees. On en derive les vitesses radiales et les mouvements spatiaux. On examine les resultats par rapport a l'appartenance a une population cinematique

UBVR observations and image-tube spectra of the faint blue star LP182-44 are analyzed. It is shown that this star is a high-space-velocity non-DA white dwarf with probable weak carbon features in its spectrum. An absolute visual magnitude of about 13.0 and a tangential velocity of approximately 190 km/s are estimated. Properties of the group of high-velocity C2 degenerates are discussed.

The spectrum of GD 190 (EG 193), a strong-lined DB white dwarf, is shown to contain an absorption feature at 4517 A in the redward wing of the He I lambda 4471 line. This feature, which is believed to be due to the forbidden 2 /sup 3/P--4 /sup 3/P transition in He I, has not been detected previously in DB white dwarfs. While its presence is predicted from model atmosphere calculations, agreement between theory and observation remains qualitative. It is possible that the lambda 4517 feature may provide a valuable gravity discriminant at least for hot DB white dwarfs. (auth)

The three X-ray selected mid-K stars with the highest values of optical flux ratios in the Einstein Medium Sensitivity Survey have been studied spectroscopically at moderate and high spectral resolution. One of these objects (1E1548.7 + 1125) is presented as a binary-star system with characteristics similar to the RS CVn class of binaries. A second of these stars (1E1751.0 + 7046) is a rapid rotator, while the third star (1E1339.9 + 6030) has not been shown to be either a binary system or a rapid rotator. The presence of both Ca II H and K emission core and high X-ray flux is attributed to chromospheric and coronal activity, but no single underlying cause is indicated.

Results of an investigation of the X-ray properties of quasars conducted using the Einstein Observatory (HEAO 2) are reported. The positions, fluxes and luminosities of 35 known quasars were observed by the Einstein high-resolution imaging detector and the imaging proportional counter. Assuming optical redshifts as valid distance indicators, 0.5-4.5 keV X-ray luminosities ranging from 10 to the 43rd to 10 to the 47 ergs/sec are obtained, with evidence of very little cold gas absorption. Flux variability on a time scale of less than 10,000 sec is observed for the quasar OX 169, which implies a mass between 8 x 10 to the 5th and 2 x 10 to the 8th solar masses for the black hole assumed to be responsible for the emission. Preliminary results of the quasar survey also indicate that quasars contribute significantly to the diffuse X-ray background.

Broad-band circular polarization measurements for 32 white dwarfs are presented, part of a continuing program to assess the magnetic field distribution and stellar parameters of magnetic white dwarfs. None of the stars discussed here represent statistically significant detections, though a few are marginal, and one in particular (LP777-1) deserves closer study. The null results for cool degenerates include the very low luminosity stars LP701-29, LP131-66, and Ross 193B (VB11), and support the hypothesis that the temperature distribution of magnetic stars is not signficantly different from the distribution of all white dwarfs. G35-26, a likely low-field case with Zeeman-split lines, does not show significant continuum circular polarization.

The triple radio source 3C 154 is centered on a 17.5 mag QSO with z=0.580. The optical spectrum is Seyfert-like. The outer radio lobes are separated by 43'', or about 375 kpc (for H/sub 0/=50, q/sub 0/=0) in projection, making 3C 154 among the larger sources in linear size. We have used a simple expansion model to determine that the characteristic outburst time scale is longer than 6 million years. (AIP)

Image-tube scans are presented for three white dwarfs known to be magnetic from polarization measurements and for the polarized recurrent nova DQ Her. Five separate scans of short duration were obtained of GD229, G240-72, and G195-19. No convincing evidence for spectral variability was found for these stars, even for the detailed band structure of GD229. The very broad (A > 1000 A) depression found by Creenstein in G240-72 is confirmed. However, for A> 4400 A no other features previously reported for G195-19 and G240-72 are confirmed by these data. The two scans of DQ Her revealed emission features reported many years ago; the underlying absorption spectrum at maximum light was featureless to A7200. Key words: white-dwarf stars - magnetic stars - novae - spectrophotometry

Results are reported for image-tube-scanner and digicon observations of Feige 7, a faint blue star identified as a probable white dwarf. It is found that this star is a magnetic white dwarf showing a very rich spectrum with Zeeman subcomponents of both hydrogen and neutral helium as well as periodic spectrum and circular-polarization variations. A polarization period of 2.2 hr is computed, and a surface magnetic-field strength of about 18 MG is determined by matching features of the absorption spectrum to Zeeman components. It is suggested that the only reasonable explanation for the periodic variations in circular polarization is an oblique rotator with the spin axis approximately in the plane of the sky and tilted by about 24 deg to the magnetic axis. An effective temperature in the range from 20,000 to 25,000 K is estimated, an absolute magnitude of about 10.5 is derived, and the atmosphere is shown to be helium-dominated. The evolution of Feige 7 is discussed in terms of possible magnetic-field effects on atmospheric composition, rotation velocity (5.5 km/s for a radius of 7000 km), and the origin of white-dwarf magnetic fields.

The Einstein Observatory's imaging proportional counter has detected 150-eV soft X-ray radiation from the four hot DA white dwarfs EG 187, Gr 288 and 289, and LB 1663. The observed pulse height spectra suggest that the emission is generated by hot photospheres whose T(eff) lie in the 30,000-60,000 K range. The IUE spacecraft UV spectra and H-beta line profiles for the four stars have been fitted, along with the X-ray fluxes, with a grid of hot, high gravity, homogeneous model atmospheres of mixed H-He composition. In all cases, the data require the presence of some X-ray opacity in the photosphere. Attention is given to the implications of this result in the context of white dwarf surface layer diffusion theories. Also noted are the limits imposed on the hot white dwarf population by the Einstein Medium Sensitivity Survey.

G35-26 is the first white dwarf to show atomic lines of both hydrogen and carbon. It should, however, be removed from the list of confirmed magnetic white dwarfs, since any surface field is less than 500,000 gauss. In accordance with the new spectral classification system of Sion et al. (1983), it may be classified as DAQ3. The star could be identified with the hot end of the helium-rich white dwarfs which reveal traces of dredged-up carbon in their atmospheres. It is noted that the absence of nitrogen or heavier element traces in the atmosphere is consistent with this hypothesis. G35-26 would be the hottest star so far to exhibit evidence for this kind of dredging, extending well into the DB temperature range. Since the star shows a high tangential velocity (Greenstein, 1978), it probably had a progenitor mass that was lower than average. In view of the evidence that the final mass may scale with the star's initial mass, the lower white-dwarf mass would allow a convective envelope of larger than normal mass fraction at 15,000 K. In this way, the layer digs deeper into the 'tail' of the carbon core abundance gradient.

A comprehensive analysis of E 1114 + 182, the first eclipsing AM Herculis binary system and the shortest-period eclipsing cataclysmic variable known, is presented. The time-resolved X-ray observations which led to the system's recognition as an AM Her system with a roughly 90 minute orbital period are reported. The current optical photometric and polarimetric ephemeris and a description of the system's phase-modulated properties are given. The detailed photometric eclipse profile and the highly variable spectroscopic behavior are addressed. This information is used to determine systemic parameters and derive new information on the line emission regions. The data put severe constraints on current torque models for keeping the binary and white dwarf rotation in phase.

Photometry and spectrophotometry of VB3, the distant companion to HR 3018, show it to be a white dwarf of low luminosity (Mv +15.6) and not an M dwarf. The pair provides the best available example of a very old white dwarf with a main-sequence star of sufficiently early type that a useful empirical limit may eventually be derivable for the ages of the oldest degenerate stars.

On discute de nouvelles observations IR entre 1,25 μm et 2,50 μm a 5% de resolution de la naine blanche froide LHS 1126. On observe une forte absorption entre 2,0 et 2,3 μm; et une absorption structuree pres de 1,6 μm

Lick image tube spectrophotometry is presented of suspected degenerate M (DM) stars and of the faintest of the suspected Hyades red subluminous (RSL) star suspects. All appear to be dwarf M's or subdwarfs. It is believed that these results, taken together with recent work by Greenstein and with Naval Observatory parallaxes, leave no remaining degenerate M or strong Hyades RSL candidates.

Image-tube scans show that the low-luminosity, proper-motion star G51-15 has an M-dwarf absorption spectrum later than Wolf 359 (dM6) and spectroscopic "flare" -star characteristics. The Balmer lines and Ca ii H and K were seen in emission, and an ultraviolet continuum excess was evident. An absorption feature at 550O-556(), recently identified with CaOH, appears very strongly in this star. Key words: spectrophotometry - late-type stars - emission-line stars - flare stars

Narrow-band photometry is presented for five stars selected from a survey of faint red stars near the north galactic pole. Four of the stars are dwarfs; the fifth is a halo giant. Spectrophotometry of the giant star supports the luminosity classification and indicates there are no strong molecular band peculiarities in the spectrum; we conclude that it lies at a distance of 10 kpc, yet has only a moderate metal deficiency, [M/Hj = -0.6 I 0.2. Key words: halo stars-photometry-spectrophotometry

Lick Observatory spectrophotometry is presented for three recognized 4670-A stars (G47-18, G99-37, and G257-38) and the DC star L1363-3. New C I features are reported for G47-18; G257-38 has no C I but definite C2; weak bands are also noted in L1363-3, which is a Wolf 219 group member. The magnetic CH star G99-37 is examined in greater detail. Model-atmosphere calculations show that it must have a helium-dominated atmosphere, since stringent upper limits on H-alpha and H-beta central Zeeman components are found. The CH 'G' band profile is compared with the theoretical calculations of Angel and Landstreet (1974). Other upper limits for atomic lines are discussed.