A Binary Orbit for the Massive, Evolved Star HDE 326823, a WR+O System Progenitor

The hot star HDE 326823 is a candidate transition-phase object that is evolving into a nitrogen-enriched Wolf-Rayet star. It is also a known low-amplitude, photometric variable with a 6.123 d period. We present new, high and moderate resolution spectroscopy of HDE 326823, and we show that the absorption lines show coherent Doppler shifts with this period while the emission lines display little or no velocity variation. We interpret the absorption line shifts as the orbital motion of the apparently brighter star in a close, interacting binary. We argue that this star is losing mass to a mass gainer star hidden in a thick accretion torus and to a circumbinary disk that is the source of the emission lines. HDE 326823 probably belongs to a class of objects that produce short-period WR+O binaries.Comment: 32 pages, 10 figures, accepted to the Astronomical Journa

A Photometric Catalogue of Southern Emission-Line Stars

We present a catalogue of previously unpublished optical and infrared photometry for a sample of 162 emission-line objects and shell stars visible from the southern hemisphere. The data were obtained between 1978 and 1997 in the Walraven (WULBV), Johnson/Cousins (UBV(RI)_c) and ESO and SAAO near-infrared (JHKLM) photometric systems. Most of the observed objects are Herbig Ae/Be (HAeBe) stars or HAeBe candidates appearing in the list of HAeBe candidates of Th\'e et al. (1994), although several B[e] stars, LBVs and T Tauri are also included in our sample. For many of the stars the data presented here are the first photo-electric measurements in the literature. The resulting catalogue consists of 1809 photometric measurements. Optical variability was detected in 66 out of the 116 sources that were observed more than once. 15 out of the 50 stars observed multiple times in the infrared showed variability at 2.2 microns (K band).Comment: 42 pages, no figures, to appear in Astronomy & Astrophysic

Galactic Twins of the Ring Nebula Around SN1987A and a Possible LBV-like Phase for Sk-69 202

Some core-collapse supernovae show clear signs of interaction with dense circumstellar material that often appears to be non-spherical. Circumstellar nebulae around supernova progenitors provide clues to the origin of that asymmetry in immediate pre-supernova evolution. Here I discuss outstanding questions about the formation of the ring nebula around SN1987A and some implications of similar ring nebulae around Galactic B supergiants. Several clues hint that SN1987A's nebula may have been ejected in an LBV-like event, rather than through interacting winds in a transition from a red supergiant to a blue supergiant.Comment: 2 pages, to appear in procedings of "Massive stars: fundamental parameters and circumstellar interactions", conference in honor of Virpi Niemela's 70th birthda

X-ray Light Curves and Accretion Disk Structure of EX Hydrae

We present X-ray light curves for the cataclysmic variable EX Hydrae obtained with the Chandra High Energy Transmission Grating Spectrometer and the Extreme Ultraviolet Explorer Deep Survey photometer. We confirm earlier results on the shape and amplitude of the binary light curve and discuss a new feature: the phase of the minimum in the binary light curve, associated with absorption by the bulge on the accretion disk, increases with wavelength. We discuss several scenarios that could account for this trend and conclude that, most likely, the ionization state of the bulge gas is not constant, but rather decreases with binary phase. We also conclude that photoionization of the bulge by radiation originating from the white dwarf is not the main source of ionization, but that it is heated by shocks originating from the interaction between the inflowing material from the companion and the accretion disk. The findings in this paper provide a strong test for accretion disk models in close binary systems.Comment: 19 pages, 4 figures, accepted for publication in the Ap

On the nature of the prototype LBV AG Carinae I. Fundamental parameters during visual minimum phases and changes in the bolometric luminosity during the S-Dor cycle

We present a detailed spectroscopic analysis of the luminous blue variable AG Carinae during the last two visual minimum phases of its S-Dor cycle (1985-1990 and 2000-2003). The analysis reveals an overabundance of He, N, and Na, and a depletion of H, C, and O, on the surface of AG Car, indicating the presence of CNO-processed material. Furthermore, the ratio N/O is higher on the stellar surface than in the nebula. We found that the minimum phases of AG Car are not equal to each other, since we derived a noticeable difference between the maximum effective temperature achieved during 1985-1990 (22,800 K) and 2000-2001 (17,000 K). While the wind terminal velocity was 300 km/s in 1985-1990, it was as low as 105 km/s in 2001. The mass-loss rate, however, was lower from 1985-1990 (1.5 x 10^(-5) Msun/yr) than from 2000-2001 (3.7 x 10^(-5) Msun/yr). We found that the wind of AG Car is significantly clumped (f=0.10 - 0.25) and that clumps must be formed deep in the wind. We derived a bolometric luminosity of 1.5 x 10^6 Lsun during both minimum phases which, contrary to the common assumption, decreases to 1.0 x 10^6 Lsun as the star moves towards maximum flux in the V band. Assuming that the decrease in the bolometric luminosity of AG Car is due to the energy used to expand the outer layers of the star (Lamers 1995), we found that the expanding layers contain roughly 0.6 - 2 Msun. Such an amount of mass is an order of magnitude lower than the nebular mass around AG Car, but is comparable to the nebular mass found around lower-luminosity LBVs and to that of the Little Homunculus of Eta Car. If such a large amount of mass is indeed involved in the S Dor-type variability, we speculate that such instability could be a failed Giant Eruption, with several solar masses never becoming unbound from the star.(abridged)Comment: 22 pages, 13 figures, ApJ in press. A high-resolution PDF version is also available at http://www.mpifr-bonn.mpg.de/staff/jgroh/agcar.htm

BRITE-Constellation reveals evidence for pulsations in the enigmatic binary η\eta Carinae

$\eta$ Car is a massive, eccentric binary with a rich observational history. We obtained the first high-cadence, high-precision light curves with the BRITE-Constellation nanosatellites over 6 months in 2016 and 6 months in 2017. The light curve is contaminated by several sources including the Homunculus nebula and neighboring stars, including the eclipsing binary CPD$-$59$^\circ$2628. However, we found two coherent oscillations in the light curve. These may represent pulsations that are not yet understood but we postulate that they are related to tidally excited oscillations of $\eta$ Car's primary star, and would be similar to those detected in lower-mass eccentric binaries. In particular, one frequency was previously detected by van Genderen et al. and Sterken et al. through the time period of 1974 to 1995 through timing measurements of photometric maxima. Thus, this frequency seems to have been detected for nearly four decades, indicating that it has been stable in frequency over this time span. These pulsations could help provide the first direct constraints on the fundamental parameters of the primary star if confirmed and refined with future observations.Comment: 8 pages, 4 figures, accepted to MNRA

Mitigation of ionospheric signatures in Swarm GPS gravity field estimation using weighting strategies

Even though ESA's three-satellite low-earth orbit (LEO) mission Swarm is primarily a magnetic field mission, it can also serve as a gravity field mission. Located in a near-polar orbit with initial altitudes of 480&thinsp;km for Swarm A and Swarm C and 530&thinsp;km for Swarm B and equipped with geodetic-type dual frequency Global Positioning System (GPS) receivers, it is suitable for gravity field computation. Of course, the Swarm GPS-only gravity fields cannot compete with the gravity fields derived from the ultra-precise Gravity Recovery And Climate Experiment (GRACE) K-band measurements. But for various reasons like the end of the GRACE mission in October 2017, data gaps in the previous months due to battery aging, and the gap between GRACE and the recently launched GRACE Follow-On mission, Swarm gravity fields became important to maintain a continuous time series and to bridge the gap between the two dedicated gravity missions. By comparing the gravity fields derived from Swarm kinematic positions to the GRACE gravity fields, systematic errors have been observed in the Swarm results, especially around the geomagnetic equator. These errors are already visible in the kinematic positions as spikes up to a few centimeters, from where they propagate into the gravity field solutions. We investigate these systematic errors by analyzing the geometry-free linear combination of the GPS carrier-phase observations and its time derivatives using a combination of a Gaussian filter and a Savitzky–Golay filter and the Rate of Total Electron Content (TEC) Index (ROTI). Based on this, we present different weighting schemes and investigate their impact on the gravity field solutions in order to assess the success of different mitigation strategies. We will show that a combination of a derivative-based weighting approach with a ROTI-based weighting approach is capable of reducing the geoid rms from 21.6 to 12.0&thinsp;mm for a heavily affected month and that almost 10&thinsp;% more kinematic positions can be preserved compared to a derivative-based screening.</p

The Infrared Continuum Sizes of Be Star Disks

We present an analysis of the near-infrared continuum emission from the circumstellar gas disks of Be stars using a radiative transfer code for a parametrized version of the viscous decretion disk model. This isothermal gas model creates predicted images that we use to estimate the HWHM emission radius along the major axis of the projected disk and the spatially integrated flux excess at wavelengths of 1.7, 2.1, 4.8, 9, and 18 ?m. We discuss in detail the effect of the disk base density, inclination angle, stellar effective temperature, and other physical parameters on the derived disk sizes and color excesses. We calculate color excess estimates relative to the stellar V -band flux for a sample of 130 Be stars using photometry from 2MASS and the AKARI infrared camera all-sky survey. The color excess relations from our models make a good match of the observed color excesses of Be stars. We also present our results on the projected size of the disk as a function of wavelength for the classical Be star ? Tauri, and we show that the model predictions are consistent with interferometric observations in the H, K', and 12 \mu m bands

Eta Carinae's Brightness Variations Since 1998: HST Observations of the Central Star

We have measured the brightness variations in Eta Carinae for the past six years using the Hubble Space Telescope Space Telescope Imaging Spectrograph and Advanced Camera for Surveys. Unlike ground-based data, observations by the HST allow direct measurement of the brightness of the central star by resolving it from the surrounding bright ejecta. We find interesting behavior during 2003 in the continuum and H$\alpha$ emission. The data show that the established long term brightening trend of Eta Car continues, including regular events which coincide with the 5.5 year spectroscopic cycle and other more rapid and unexpected variations. In addition to the HST data, we also present ground-based data obtained from the AAVSO which show many of the same features. The dip in the apparent brightness of the central star at the time of the 2003.5 event is wavelength dependent with no decrease in the continuum. These observations cast doubt on asimple eclipse or occultation as the explanation for the dip and place constraints on the models for the event.Comment: 28 pages, 9 Figures, 3 Tables. Accepted for publication in the AJ (April 2004