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Register a new usergamma Cas stars: Normal Be stars with disks impacted by the wind of a helium-star companion?
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$gamma$ Cas stars are a $sim$1% minority among classical Be stars with hard but only moderately strong continuous thermal X-ray flux and mostly very early-B spectral type. The X-ray flux has been suggested to originate from matter accelerated via magnetic disk-star interaction, by a rapidly rotating neutron star (NS) companion via the propeller effect, or by accretion onto a white dwarf (WD) companion. In view of the growing number of identified $gamma$ Cas stars and the only imperfect matches between these suggestions and the observations, alternative models should be pursued. Two of the three best-observed $gamma$ Cas stars, $gamma$ Cas itself and $pi$ Aqr, have a low-mass companion with low optical flux; interferometry of BZ Cru is inconclusive. Binary-evolution models are examined for their ability to produce such systems. The OB+He-star stage of post-mass transfer binaries, which is otherwise observationally unaccounted, can potentially reproduce many observed properties of $gamma$ Cas stars. The interaction of the fast wind of helium stars with the disk and/or with the wind of Be stars may give rise to the production of hard X-rays. While not modelling this process, it is shown that the energy budget is favourable, and that the wind velocities may lead to hard X-rays as observed in $gamma$ Cas stars. Furthermore, their observed number appears to be consistent with the evolutionary models. Within the Be+He-star binary model, the Be stars in $gamma$ Cas stars are conventional classical Be stars. They are encompassed by O-star+Wolf-Rayet systems towards higher mass, where no stable Be decretion disks exist, and by Be+sdO systems at lower mass where the sdO winds may be too weak to cause the $gamma$ Cas phenomenon. In decreasing order of the helium-star mass, the descendants could be Be+black-hole, Be+NS or Be+WD binaries.
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In the last years, a peculiarity of some Be stars - their association with unusually hard and intense X-ray emission - was shown to extend beyond a mere few cases. In this paper, we continue our search for new cases by performing a limited survey of 18 Be stars using XMM-Newton. The targets were selected either on the basis of a previous X-ray detection (Exosat, ROSAT, XMM-slew survey) without spectral information available, or because of the presence of a peculiar spectral variability. Only two targets remain undetected in the new observations and three other stars only display faint and soft X-rays. Short-term and/or long-term variations were found in one third of the sample. The spectral characterization of the X-ray brightest 13 stars of the sample led to the discovery of three new gamma Cas (HD44458, HD45995, V558Lyr), bringing the total to 25 known cases, and another gamma Cas candidate (HD120678), bringing the total to 2.
We investigate the short-term optical variability of two gamma Cas analogs, pi Aqr and BZ Cru, thanks to intensive ground-based spectroscopic and space-borne photometric monitorings. For both stars, low-amplitude (mmag) coherent photometric variability is detected. The associated signals display long-term amplitude variations, as in other Be stars. However, these signals appear at high frequencies, especially in pi Aqr, indicating p-modes with a high degree l, a quite unusual feature amongst Be stars. While BZ Cru presents only low-level spectral variability, without clear periodicity, this is not the case of pi Aqr. In this star, the dominant photometric frequencies, near ~12/d, are confirmed spectroscopically in separate monitorings taken during very different disk activity levels ; the spectroscopic analysis suggests a probable tesseral nature for the mode.
gamma-Cas stars constitute a subgroup of Be stars showing unusually hard and bright X-ray emission. In search for additional peculiarities, we analyzed the TESS lightcurves of 15 gamma-Cas analogs. Their periodograms display broad frequency groups and/or narrow isolated peaks, often superimposed over red noise. The detected signals appear at low frequencies, with few cases of significant signals beyond 5/d (and all of them are faint). The signal amplitudes, and sometimes the frequency content, change with time, even in the absence of outburst events. On the basis of their optical photometric variability, gamma-Cas stars reveal no distinctive behaviour and thus appear similar to Be stars in general.
Thermonuclear supernovae (SNe), a subset of which are the highly important SNe Type,Ia, remain one of the more poorly understood phenomena known to modern astrophysics. In recent years, the single degenerate helium (He) donor channel, where a white dwarf star (WD) accretes He-rich matter from a hydrogen-depleted companion, has emerged as a promising candidate progenitor scenario for these events. An unresolved question in this scenario is the fate of the companion star, which would be evident as a runaway hot subdwarf (He sdO/B) in the aftermath of the SN event. Previous studies have shown that the kinematic properties of an ejected companion provide an opportunity to closer examination of the properties of an SN progenitor system. However, with the number of observed objects not matching predictions by theory, the viability of this mechanism is called into question. In this study, we first synthesize a population of companion stars ejected by the aforementioned mechanism, taking into account predicted ejection velocities, inferred population density in the Galactic (Gal.) mass distribution and subsequent kinematics in the Gal. potential. We then discuss the astrometric properties of this population. We present $10^{6}$ individual ejection trajectories, numerically computed with a newly developed, lightweight simulation framework. A peak in the density distribution for close objects is expected in the direction of the Gal. center. If the entire considered mass range is realized, the radial velocity distribution should show a peak at 500kms. If only close US,708 analogues are considered, there should be a peak at ($sim750-850$)kms. We show that the puzzling lack of confirmed surviving companion stars of thermonuclear SNe, though possibly an observation-related selection effect, may indicate a selection against high mass donors in the SD He donor channel. (-abridged-)
In photometry of $gamma$ Cas (B0.5 IVe) from the SMEI and BRITE-Constellation satellites, indications of low-order non-radial pulsation have recently been found, which would establish an important commonality with the class of classical Be stars at large. New photometry with the TESS satellite has detected three frequency groups near 1.0 ($g1$), 2.4 ($g2$), and 5.1 ($g3$) d$^{-1}$, respectively. Some individual frequencies are nearly harmonics or combination frequencies but not exactly so. Frequency groups are known from roughly three quarters of all classical Be stars and also from pulsations of $beta$ Cep, SPB, and $gamma$ Dor stars and, therefore, firmly establish $gamma$ Cas as a non-radial pulsator. The total power in each frequency group is variable. An isolated feature exists at 7.57 d$^{-1}$ and, together with the strongest peaks in the second and third groups ordered by increasing frequency ($g2$ and $g3$), is the only one detected in all three TESS sectors. The former long-term 0.82 d$^{-1}$ variability would fall into $g1$ and has not returned at a significant level, questioning its attribution to rotational modulation. Low-frequency stochastic variability is a dominant feature of the TESS light curve, possibly caused by internal gravity waves excited at the core-envelope interface. These are known to be efficient at transporting angular momentum outward, and may also drive the oscillations that constitute $g1$ and $g2$. The hard X-ray flux of $gamma$ Cas is the only remaining major property that distinguishes this star from the class of classical Be stars.
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