Richard A. Wade

The Neptune-sized Circumbinary Planet Kepler-38b

We discuss the discovery and characterization of the circumbinary planet Kepler-38b. The stellar binary is single-lined, with a period of 18.8 days, and consists of a moderately evolved main-sequence star (M_A = 0.949 ± 0.059 M_☉ and R_A = 1.757 ± 0.034 R_☉) paired with a low-mass star (M_B = 0.249 ± 0.010 M_☉ and R_B = 0.2724 ± 0.0053 R_☉) in a mildly eccentric (e = 0.103) orbit. A total of eight transits due to a circumbinary planet crossing the primary star were identified in the Kepler light curve (using Kepler Quarters 1-11), from which a planetary period of 105.595 ± 0.053 days can be established. A photometric dynamical model fit to the radial velocity curve and Kepler light curve yields a planetary radius of 4.35 ± 0.11 R_⊕, or equivalently 1.12 ± 0.03 R_(Nep). Since the planet is not sufficiently massive to observably alter the orbit of the binary from Keplerian motion, we can only place an upper limit on the mass of the planet of 122 M_⊕ (7.11 M_(Nep) or equivalently 0.384 M_(Jup)) at 95% confidence. This upper limit should decrease as more Kepler data become available.

PG0027 + 260 - An example of a class of cataclysmic binaries with mysterious, but consistent, behavior

This paper reports extensive optical observations on the PG0027 + 260 binary, carried out on August 1984 with the 1.3 McGraw-Hill telescope and Mark II spectrometer at Michigan-Dartmouth-MIT Observatory on Kitt Peak. It is shown that this object is an eclipsing novalike variable with an orbital period of 3.51 hr. The PG0027 + 260 displays several unexplained phenomena which are remarkably similar to those of the SW Sex, DW UMa, and V1315 Aql, which are eclipsing novalike stars with periods between 3 and 4 hrs. The eclipse of the PG0027 + 260 is modeled, and it is shown that, while the mean eclipse light curve is easy to match, there is no simple explanation for the variable depth. 58 refs.

Confirmation of eclipses in KPD 0422+5421, a binary containing a white dwarf and a subdwarf B star

We report additional photometric CCD observations of KPD 0422+5421, a binary with an orbital period of 2.16 hours which contains a subdwarf B star (sdB) and a white dwarf. There are two main results of this work. First, the light curve of KPD 0422+5421 contains two distinct periodic signals, the 2.16 hour ellipsoidal modulation discovered by Koen, Orosz, & Wade (1998) and an additional modulation at 7.8 hours. This 7.8 hour modulation is clearly not sinusoidal: the rise time is about 0.25 in phase, whereas the decay time is 0.75 in phase. Its amplitude is roughly half of the amplitude of the ellipsoidal modulation. Second, after the 7.8 hour modulation is removed, the light curve folded on the orbital period clearly shows the signature of the transit of the white dwarf across the face of the sdB star and the signature of the occultation of the white dwarf by the sdB star. We used the Wilson-Devinney code to model the light curve to obtain the inclination, the mass ratio, and the potentials, and a Monte Carlo code to compute confidence limits on interesting system parameters. We find component masses of MsdB = 0.36 +0.37 −0.16 M⊙ and MWD = 0.47 +0.18 −0.16 M⊙ (Mtotal = 0.86 +0.52 −0.35 M⊙, 68 per cent confidence limits). If we impose an additional constraint and require the computed mass and radius of the white dwarf to be consistent with a theoretical mass-radius relation, we find MsdB = 0.511 +0.047 −0.050 M⊙ and MWD = 0.526 +0.033 −0.030 M⊙ (68 per cent confidence limits). In this case the total mass of the system is less than 1.4 M⊙ at the 99.99 per cent confidence level. We briefly discuss possible interpretations of the 7.8 hour modulation and the importance of KPD 0422+5421 as a member of a rare class of evolved binaries.

OBSERVATIONS OF THE BRIGHT NOVALIKE VARIABLE IX VELORUM WITH THE HOPKINS ULTRAVIOLET TELESCOPE

The Hopkins Ultraviolet Telescope, an experiment flown on the Space Shuttle as part of the Astro-1 mission, was used to obtain a spectrum of the novalike variable IX Vel (= CPD -48 deg 1577) in the wavelength range 830-1860 A. The observation revealed a rich absorption-line and continuum spectrum that peaks near 1050 A at a flux of 1.6 x 10(exp -11) ergs/sq cm/s/A. In the sub-Lyman-alpha region, some of the more prominent absorption lines are S VI lambda lambda-933, 945, C III lambda-977, Lyman-beta, O VI lambda lambda-1032, 1038, P V lambda lambda-1118, 1128, and C III lambda-1176. No emission was detected below the Lyman limit. The overall continuum shape of IX Vel in the FUV can be approximated using models of an optically thick accretion disk in which the integrated spectrum has been constructed by summing model stellar atmospheres or proper disk model spectra. However, if the distance to IX Vel is approximately 95 pc, standard disk models without reddening cannot simultaneously reproduce the color and flux in the UV. While interstellar reddening can reconcile this difference, the amount of reddening appears inconsistent with the absence of a 2200 A bump in the spectrum and the very low H I column density measured along the line of sight. Improved fits to the data can be obtained by modifying the accretion disk stucture within three white dwarf radii. None of the models reproduces the profiles of the Li- and Na-like ions, which are observed as strong but relatively narrow absorption lines, and which are almost surely due to a wind above the disk.

KPD 0422+5421: a new short-period subdwarf B/white dwarf binary

The sdB star KPD 0422+5421 was discovered to be a single-lined spectroscopic binary with a period of P = 0.0901795±(3×10 −7 ) days (2 hours, 10 minutes). The U and B light curves display an ellipsoidal modulation with amplitudes of � 0.02 magnitudes. The sdB star contributes nearly all of the observed flux. This and the absence of any reflection effect suggest that the unseen companion star is small (i.e. Rcomp � 0.01 R⊙) and therefore degenerate. We modeled the U and B light curves and derived i = 78.05±0.50 ◦ and a mass ratio of q = Mcomp/MsdB = 0.87±0.15. The sdB star fills 69% of its Roche lobe. These quantities may be combined with the mass function of the companion (f(M) = 0.126±0.028 M⊙) to derive MsdB = 0.72±0.26 M⊙ and Mcomp = 0.62 ± 0.18 M⊙. We used model spectra to derive the effective temperature, surface gravity, and helium abundance of the sdB star. We found Teff = 25,000 ± 1500 K, log g = 5.4 ± 0.1, and [He/H] = 1.0. With a period of 2 hours and 10 minutes, KPD 0422+5421 has one of the shortest known orbital periods of a detached binary. This system is also one of only a few known binaries which contain a subdwarf B star and a white dwarf. Thus KPD 0422+5421 represents a relatively unobserved, and short-lived, stage of binary star evolution.

Near-infrared spectra of Seyfert nuclei. I. The reddening problem

IR emission-line fluxes of various species from recent J and H band observations of five active galactic nuclei are given. These include a quasar and a Seyfert type 1, 1.5, 1.9, and 2. The infrared-to-optical ratios are used to derive the reddening appropriate to the regions emitting the broad and narrow emission lines, and comparison is made with other methods. The line ratio He II 3203 A to He II of 1.0123 microns is proposed as a useful new reddening diagnostic and is used to determine the A(v) in NGC 1068 and NGC 4151. The broad line extinction in NGC 2922 is inferred for the first time, and considered in relation to its X-ray properties. 45 references.

PG 0308 + 096 and PG 1026 + 002 - Two new short period binary stars resulting from common-envelope evolution

Ultraviolet spectroscopy, optical spectroscopy, and spectrophotometry have been used to study the excess UV stars PG 0308 + 096 and PG 1026 + 002. Both objects are short-period binary systems, each containing a DA white dwarf star and a dM star. Orbital periods of approximately 0.284 day for PG 0308 + 096, and aproximately 0.597 day for PG 1026, have been found by spectroscopic analysis of the H-alpha emission line. Ly-alpha and Balmer line profile fitting were used to estimate the mass of white dwarf stars; mass estimates for the dM stars are based on their spectral types. The orbital inclinations are derived from these masses, the periods, and amplitudes of the H-alpha radial velocity curves. The equivalent width of the H-alpha emission line, in each binary system, varies with the orbital phase in such a manner as to imply that it arises, in large part at least, from the hemisphere of the M star that faces the white dwarf star.

PG 1018−047: the longest period subdwarf B binary

About 50 per cent of all known hot subdwarf B stars (sdBs) reside in close (short-period) binaries, for which common-envelope ejection is the most likely formation mechanism. However, Han et al. predict that the majority of sdBs should form through stable mass transfer leading to long-period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of ∼100 short-period systems have been measured, there are no periods measured above 30 d. As part of a large programme to characterize the orbital periods of sdB binaries and their formation history, we have found that PG 1018−047 has an orbital period of 759.8 ± 5.8 d, easily making it the longest period ever detected for a sdB binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio MMS/MsdB = 1.6 ± 0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid-K.

Recovery of 29 Second Oscillations in Hubble Space Telescope Eclipse Observations of the Cataclysmic Variable UX Ursae Majoris

Low-amplitude (0.5%) 29 s oscillations have been detected in Hubble Space Telescope Faint Object Spectrograph eclipse observations of the nova-like cataclysmic variable UX UMa. These are the same dwarf nova-type oscillations that were originally discovered in this system by Warner & Nather in 1972. The 29 s oscillations are seen in one pair of eclipse sequences obtained with the FOS/PRISM in 1994 November but not in a similar pair obtained with the FOS/G160L grating in August of the same year. The oscillations in the PRISM data are sinusoidal to within the small observational errors and undergo an approximately -360° phase shift during eclipses (i.e., one cycle is lost). The amplitudes are highest at preeclipse orbital phases and exhibit a rather gradual eclipse whose shape is roughly similar to, although perhaps slightly narrower than, UX UMas overall light curve in the PRISM bandpass (2000-8000 A). Spectra of the oscillations have been constructed from pre-, mid, and posteclipse data segments of the November observations. The spectra obtained from the out-of-eclipse segments are extremely blue, and only lower limits can be placed on the temperature of the source that dominates the modulated flux at these orbital phases. Lower limits derived from blackbody (stellar atmosphere) model fits to these data are ≥95,000 K (≥85,000 K); the corresponding upper limits on the projected area of this source are all less than 2% of the white dwarf (WD) surface area. By contrast, oscillation spectra derived from mideclipse data segments are much redder. Fits to these spectra yield temperature estimates in the range 20,000 K T 30,000 K for both blackbody and stellar atmosphere models and corresponding projected areas of a few percent of the WD surface area. These estimates are subject to revision if the modulated emission is optically thin. We suggest that the ultimate source of the oscillations is a hot, compact region near disk center, but that a significant fraction of the observed, modulated flux is due to reprocessing of the light emitted by this source in the accretion disk atmosphere. The compact source is occulted at orbital phases near mideclipse, leaving only part of the more extended reprocessing region(s) to produce the weak oscillations that persist even at conjunction. The highly sinusoidal oscillation pulse shape does not permit the identification of the compact component in this model with emission produced by a rotating disturbance in the inner disk or in a classical, equatorial boundary layer. Instead, this component could arise in a bright spot on the surface of the WD, possibly associated with a magnetic pole. However, a standard intermediate polar model can also be ruled out since UX UMas oscillation period has been seen to change on timescales much shorter than the minimum timescale required to spin up the WD by accretion torques. A model invoking magnetically controlled accretion onto differentially rotating WD surface layers may be viable, but needs more theoretical work.

POPULATION SYNTHESIS OF HOT SUBDWARFS: A PARAMETER STUDY

Binaries that contain a hot subdwarf (sdB) star and a main-sequence companion may have interacted in the past. This binary population has historically helped determine our understanding of binary stellar evolution. We have computed a grid of binary population synthesis models using different assumptions about the minimum core mass for helium ignition, the envelope binding energy, the common-envelope ejection efficiency, the amount of mass and angular momentum lost during stable mass transfer, and the criteria for stable mass transfer on the red giant branch and in the Hertzsprung gap. These parameters separately and together can significantly change the entire predicted population of sdBs. Nonetheless, several different parameter sets can reproduce the observed subpopulation of sdB + white dwarf and sdB + M dwarf binaries, which has been used to constrain these parameters in previous studies. The period distribution of sdB + early F dwarf binaries offers a better test of different mass transfer scenarios for stars that fill their Roche lobes on the red giant branch.