Publications of the Astronomical Society of the Pacific

Merritt, David

doi: 10.1086/316307pmid: N/A

Torres, Guillermo

doi: 10.1086/316313pmid: N/A

Slow drifts detected in the radial velocities of a number of nearby stars indicate the presence of companions in orbit around the primaries, even though a complete spectroscopic orbital solution is not possible because of the long periods. Some of these companions may have substellar masses and are thus of considerable interest for the study of the characteristics of extrasolar planets and brown dwarfs. Similar indications can be obtained from the measurement of small astrometric accelerations of the primary star, either from the ground or more likely from future highly precise space astrometry missions.We show here that a single measurement of the angular separation of such companions, if they can be imaged directly, provides valuable statistical information on their mass when combined with the accelerations. We derive the probability distribution of the companion mass separately for the radial velocity case and for the astrometric case by Monte Carlo simulations, and illustrate the application of the technique with several examples. The information on the masses of such wide substellar companions is complementary to that from closer spectroscopic companions, and a full analysis of the mass distribution of these objects should include both to avoid possible biases.

Hastings, N. C.; Szkody, Paula; Hoard, D. W.; Fried, Robert; Vanmunster, Tonny; Pray, Don; Kowalski, R. A.

doi: 10.1086/316318pmid: N/A

We observed V884 Herculis (RX J1802.1+1804), during a high accretion state, from 1997 September 17–22 UT using the Extreme Ultraviolet Explorer (EUVE) satellite. Simultaneously, multiple ground‐based optical telescopes obtained photometric observations. Although the EUV flux was too low for spectral study, the light curve from the Deep Survey instrument provides useful information on the system geometry. The phased, folded EUV light curve shows a brief total eclipse similar to previous ROSAT PSPC data followed by a broad dip. The optical light curve shows a lower amplitude of variability and a smoother structure. The similarity of the optical light curves on all six nights, along with previous magnetic measurements, and the coincidence of the periods determined from the optical and EUVE data, confirm the synchronism of this system and its classification as an AM Her type magnetic cataclysmic variable. Doppler tomography of 1995 high state spectra reveals a short, ballistic accretion stream with little emission from an irradiated secondary star. An optical light curve obtained in 1998 June shows the system during a lowered accretion state, with a mean magnitude that is ∼1.5 mag fainter than the high state. The low state light curve displays a smaller amplitude of orbital variation but only slight changes in accretion geometry. A spectrum obtained 15 days later shows that the optical continuum had returned to a high state, although the line strengths appear different from past high states.

Taylor, Cynthia J.; Thorstensen, John R.; Patterson, Joseph

doi: 10.1086/316316pmid: N/A

We present time‐resolved spectroscopy and photometry of the bright cataclysmic variable LS Peg (=S193; V ≈ 13.0—Szkody et al.). The Balmer lines exhibit broad, asymmetric wings Doppler‐shifted by about 2000 km s−1 at the edges, while the He i lines show phase‐dependent absorption features strikingly similar to SW Sextantis stars, as well as emission through most of the phase. The Ciii/N iii emission blend does not show any phase dependence. From velocities of Hα emission lines, we determine an orbital period of 0.174774 ± 0.000003 days (=4.1946 hr), which agrees with Szkody's value of approximately 4.2 hr. No stable photometric signal was found at the orbital period. A noncoherent quasi‐periodic photometric signal was seen at a period of 20.7 ± 0.3 minutes.The high‐velocity Balmer wings most probably arise from a stream reimpact point close to the white dwarf. We present simulated spectra based on a kinematic model similar to the modified disk‐overflow scenario of Hellier & Robinson. The models reproduce the broad line wings, though some other details are unexplained.Using an estimate of dynamical phase based on the model, we show that the phasing of the emission‐ and absorption‐line variations is consistent with that in (eclipsing) SW Sex stars. We therefore identify LS Peg as a low‐inclination SW Sex star.Our model suggests i = 30°, and the observed absence of any photometric signal at the orbital frequency establishes i<60°. This constraint puts a severe strain on interpretations of the SW Sex phenomenon which rely on disk structures lying slightly out of the orbital plane.

Davis, Cecil G.

doi: 10.1086/316308pmid: N/A

The methods of Davis & Cox are applied to a series of models described by Bono & Stellingwerf to determine the colors of RR Lyrae stars. Convection is ignored, and the radiation flow is treated by a complete variable Eddington, multifrequency‐dependent radiative transfer approximation.

Percy, John R.; Bagby, Dalia H.

doi: 10.1086/316314pmid: N/A

The maximum magnitudes of many Mira stars fluctuate from cycle to cycle. In some cases, this may be due to multiperiodicity. We have analyzed the maximum magnitudes of a large sample of bright Mira stars, using a portion of the American Association of Variable Star Observers international database of variable star observations: times and magnitudes of maximum and minimum of 391 bright Mira stars over 75 years. We find many whose behavior can be interpreted as due to multiperiodicity, with either a secondary period that is an order of magnitude longer than the primary period or a secondary period comparable with the primary one. In the former case, the nature of the long secondary period is not clear, although, in some cases, it may be related to episodic dust emission. In the latter case, the behavior is most consistent with the hypothesis that the primary period is the first overtone period and the secondary period is another radial mode.

Patten, Brian M.; Pavlovsky, Cheryl M.

doi: 10.1086/316315pmid: N/A

A VRI photometric survey has been conducted in the young open cluster IC 2391 to identify candidate low‐mass cluster members. Photometry for some 10,725 objects, ranging from V = 10 to V = 20, were extracted from 21 CCD fields positioned within 1° of the nominal cluster center[α(2000) = 08h42m, δ(2000) = -53°00′]. Seventeen stars have been identified as candidate cluster members with (V−I) colors ranging from 2.78 to 3.72. The masses of these candidate members are estimated to range from 0.25 to 0.09 M⊙. If these stars are indeed IC 2391 members, they will help to rectify an apparent deficit of low‐luminosity stars in the luminosity function of this cluster.

Bond, Howard E.; Ciardullo, Robin

doi: 10.1086/316321pmid: N/A

The central star of the planetary nebula NGC 246 is a resolved visual binary. We present CCD photometry of the G8–K0 V companion, which is located 3.″8 from the hot nucleus. By fitting the companion to the zero‐age main sequence, we derive a distance of d = 495+145-100 pc. This is one of the more secure distances known for a Galactic planetary nebula, but it could be improved still further with a better estimate of the metallicity of the system.

Feibelman, Walter A.

doi: 10.1086/316317pmid: N/A

We present the first detection of O vii λ1522 emission or absorption from archival IUE spectra in 14 planetary nebula nuclei and three PG 1159–type stars. The n = 5→6 transition of O vii was determined by Kruk & Werner and observed by them in the spectrum of the very hot PG 1159–type star H1504+65 from data obtained with the Hopkins Ultraviolet Telescope (HUT). Emission‐line fluxes or absorption equivalent widths as well as radial velocities for the program stars are presented. The precise rest wavelength for the 5→6 transition requires further investigation.

Lauer, Tod R.

doi: 10.1086/316319pmid: N/A

Undersampled images, such as those produced by the HST WFPC‐2, misrepresent fine‐scale structure intrinsic to the astronomical sources being imaged. Analyzing such images is difficult on scales close to their resolution limits and may produce erroneous results. A set of “dithered” images of an astronomical source generally contains more information about its structure than any single undersampled image, however, and may permit reconstruction of a “superimage” with Nyquist sampling. I present a tutorial on a method of image reconstruction that builds a Nyquist superimage from a complex linear combination of the Fourier transforms of a set of undersampled dithered images. This method works by algebraically eliminating the high‐order satellites in the periodic transforms of the aliased images. The reconstructed image is an exact representation of the data set with no loss of resolution at the Nyquist scale. The algorithm is directly derived from the theoretical properties of aliased images and involves no arbitrary parameters, requiring only that the dithers are purely translational and constant in pixel space over the domain of the object of interest. I show examples of its application to WFC and PC images. I argue for its use when the best recovery of point sources or morphological information at the HST diffraction limit is of interest.