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1. Mass limits of the extremely fast-spinning white dwarf CTCV J2056–3014

   https://doi.org/10.1051/0004-6361/202039749  

   Otoniel, Edson ; Coelho, Jaziel G. ; Nunes, Sílvia P. ; Malheiro, Manuel ; Weber, Fridolin ( December 2021 , Astronomy & Astrophysics)

   CTCV J2056–3014 is a nearby cataclysmic variable with an orbital period of approximately 1.76 h at a distance of about 853 light-years from the Earth. Its recently reported X-ray properties suggest that J2056–3014 is an unusual accretion-powered intermediate polar that harbors a fast-spinning white dwarf (WD) with a spin period of 29.6 s. The low X-ray luminosity and the relatively modest accretion rate per unit area suggest that the shock is not occurring near the WD surface. It has been argued that, under these conditions, the maximum temperature of the shock cannot be directly used to determine the mass of the WD (which, under the abovementioned assumptions, would be around 0.46 M ⊙ ). Here, we explore the stability of this rapidly rotating WD using a modern equation of state (EoS) that accounts for electron–ion, electron–electron, and ion–ion interactions. For this EoS, we determine the mass density thresholds for the onset of pycnonuclear fusion reactions and study the impact of microscopic stability and rapid rotation on the structure and stability of WDs, considering them with helium, carbon, oxygen, and neon. From this analysis, we obtain a minimum mass for CTCV J2056–3014 of 0.56 M ⊙ and a maximum mass ofmore »around 1.38 M ⊙ . If the mass of CTCV J2056–3014 is close to the lower mass limit, its equatorial radius would be on the order of 10 4 km due to rapid rotation. Such a radius is significantly larger than that of a nonrotating WD of average mass (0.6  M ⊙ ), which is on the order of 7 × 10 3 km. The effects on the minimum mass of J2056–3014 due to changes in the temperature and composition of the stellar matter were found to be negligibly small.« less
2. Gamma Cas Stars as Be+White Dwarf Binary Systems

   https://doi.org/10.3847/2041-8213/acaaa1  

   Gies, Douglas R. ; Wang, Luqian ; Klement, Robert ( December 2022 , The Astrophysical Journal Letters)

   The origin of the bright and hard X-ray emission flux among theγCas subgroup of B-emission line (Be) stars may be caused by gas accretion onto an orbiting white dwarf (WD) companion. Such Be+WD binaries are the predicted outcome of a second stage of mass transfer from a helium star mass donor to a rapidly rotating mass gainer star. The stripped donor stars become small and hot white dwarfs that are extremely faint compared to their Be star companions. Here we discuss model predictions about the physical and orbital properties of Be+WD binaries, and we show that current observational results onγCas systems are consistent with the expected large binary frequency, companion faintness and small mass, and relatively high mass range of the Be star hosts. We determine that the companions are probably not stripped helium stars (hot subdwarf sdO stars), because these are bright enough to detect in ultraviolet spectroscopy, yet their spectroscopic signatures are not observed in studies ofγCas binaries. Interferometry of relatively nearby systems provides the means to detect very faint companions including hot subdwarf and cooler main-sequence stars. Preliminary observations of fiveγCas binaries with the CHARA Array interferometer show no evidence of the companion flux, leaving whitemore »dwarfs as the only viable candidates for the companions.

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3. Signatures of progenitors of Type Ia supernovae

   P. Hoeflich, S. Chakraborty ( October 2017 , Memorie della Società astronomica italiana)

   Thermonuclear Supernovae (SNe Ia) are one of the building blocks of modern cosmology and laboratories for the explosion physics of White Dwarf star/s (WD) in close binary systems. The second star may be aWD(double degenerate systems, DD), or a non-degenerated star (SD) with a main sequence star, red giant or a helium star as companion (Branch et al. 1995; Nomoto et al. 2003; Wang & Han 2012). Light curves and spectra of the explosion look similar because a ’stellar amnesia’ (H¨oflich et al. 2006). Basic nuclear physics determines the progenitor structure and the explosion physics, breaking the link between progenitor evolution, and the explosion, resulting in three main classes of explosion scenarios: a) dynamical merging of two WD and a heating on time scales of seconds (Webbink 1984; Isern et al. 2011), b) surface helium detonations on top of a WD which ignite the central C/O by a detonation wave traveling inwards (Nomoto 1982; Hoeflich & Khokhlov 1996; Kromer et al. 2010); c) compressional heating in an accreting WD approaching the Chandrasekar mass on time of up to 108 years which may originated from SD and DD systems (Whelan & Iben 1973; Piersanti et al. 2003). Simulations of the explosionsmore »depend on the inital conditions at the onset of the explosions, namely the mass and angular momentum of the WD(s). For all scenarios, diversity in SNe Ia must be expected because the WD originates from a range of Main Sequence masses (MMS < 8M ) and metallicities Z. Moreover, there is growing evidence that magnetic fields B may have to be added to the ’mix’. Only with recent advances in observations ranging from X-ray to radio, high precision spectroscopy, polarimetry and photometry and in the time-domain astronomy we obtain constraints for progenitor, on the explosion scenarios and links emerge between the progenitors and their environment with LCs and spectral signatures needed for high precision cosmology. It is too early to give final answers but we present our personal view. We will give some examples from the theory point of view and discuss future prospects with upcoming ground based, ELT, GMT and space based such as JWST, Euclide and WFIRST instruments.« less
4. Measuring He i Stark Line Shapes in the Laboratory to Examine Differences in Photometric and Spectroscopic DB White Dwarf Masses

   https://doi.org/10.3847/1538-4357/ac9df5  

   Schaeuble, M. -A. ; Nagayama, T. ; Bailey, J. E. ; Gigosos, M. A. ; Florido, R. ; Blouin, S. ; Gomez, T. A. ; Dunlap, B. ; Montgomery, M. H. ; Winget, D. E. ( December 2022 , The Astrophysical Journal)

   Accurate helium White Dwarf (DB) masses are critical for understanding the star’s evolution. DB masses derived from the spectroscopic and photometric methods are inconsistent. Photometric masses agree better with currently accepted DB evolutionary theories and are mostly consistent across a large range of surface temperatures. Spectroscopic masses rely on untested HeiStark line-shape and Van der Waals broadening predictions, show unexpected surface temperature trends, and are thus viewed as less reliable. To test this conclusion, we present in this paper detailed HeiStark line-shape measurements at conditions relevant to DB atmospheres (T_electron≈12,000–17,000 K,n_electron≈ 10¹⁷cm⁻³). We use X-rays from Sandia National Laboratories’Z-machine to create a uniform ≈120 mm long hydrogen–helium mixture plasma. Van der Waals broadening is negligible at our experimental conditions, allowing us to measure HeiStark profiles only. Hβ, which has been well-studied in our platform and elsewhere, serves as then_ediagnostic. We find that HeiStark broadening models used in DB analyses are accurate within errors at tested conditions. It therefore seems unlikely that line-shape models are solely responsible for the observed spectroscopic mass trends. Our results should motivate the WD community to further scrutinize the validity of other spectroscopic and photometric input parameters, like atmospheric structure assumptions and convection corrections. Thesemore »parameters can significantly change the derived DB mass. Identifying potential weaknesses in any input parameters could further our understanding of DBs, help elucidate their evolutionary origins, and strengthen confidence in both spectroscopic and photometric masses.

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5. ALMA multiline survey of the ISM in two quasar host–companion galaxy pairs at z > 6

   https://doi.org/10.1051/0004-6361/202039696  

   Pensabene, A. ; Decarli, R. ; Bañados, E. ; Venemans, B. ; Walter, F. ; Bertoldi, F. ; Fan, X. ; Farina, E. P. ; Li, J. ; Mazzucchelli, C. ; et al ( August 2021 , Astronomy & Astrophysics)

   We present a multiline survey of the interstellar medium (ISM) in two z  > 6 quasar host galaxies, PJ231−20 ( z  = 6.59) and PJ308−21 ( z  = 6.23), and their two companion galaxies. Observations were carried out using the Atacama Large (sub-)Millimeter Array (ALMA). We targeted 11 transitions including atomic fine-structure lines (FSLs) and molecular lines: [NII] 205 μm , [CI] 369 μm , CO ( J up  = 7, 10, 15, 16), H 2 O 3 12  − 2 21 , 3 21  − 3 12 , 3 03  − 2 12 , and the OH 163 μm doublet. The underlying far-infrared (FIR) continuum samples the Rayleigh-Jeans tail of the respective dust emission. By combining this information with our earlier ALMA [CII] 158 μm observations, we explored the effects of star formation and black hole feedback on the ISM of the galaxies using the CLOUDY radiative transfer models. We estimated dust masses, spectral indexes, IR luminosities, and star-formation rates from the FIR continuum. The analysis of the FSLs indicates that the [CII] 158 μm and [CI] 369 μm emission arises predominantly from the neutral medium in photodissociation regions (PDRs). We find that line deficits agree with those of local luminous IR galaxies. The CO spectral line energy distributions (SLEDs) reveal significant high- J COmore »excitation in both quasar hosts. Our CO SLED modeling of the quasar PJ231−20 shows that PDRs dominate the molecular mass and CO luminosities for J up  ≤ 7, while the J up  ≥ 10 CO emission is likely driven by X-ray dissociation regions produced by the active galactic nucleus (AGN) at the very center of the quasar host. The J up  > 10 lines are undetected in the other galaxies in our study. The H 2 O 3 21  − 3 12 line detection in the same quasar places this object on the L H 2 O  −  L TIR relation found for low- z sources, thus suggesting that this water vapor transition is predominantly excited by IR pumping. Models of the H 2 O SLED and of the H 2 O-to-OH 163 μm ratio point to PDR contributions with high volume and column density ( n H  ∼ 0.8 × 10 5 cm −3 , N H  = 10 24 cm −2 ) in an intense radiation field. Our analysis suggests a less highly excited medium in the companion galaxies. However, the current data do not allow us to definitively rule out an AGN in these sources, as suggested by previous studies of the same objects. This work demonstrates the power of multiline studies of FIR diagnostics in order to dissect the physical conditions in the first massive galaxies emerging from cosmic dawn.« less