ASASSN-16oh: A Nova Outburst with No Mass Ejection—A New Type of Supersoft X-Ray Source in Old Populations

Abstract

ASASSN-16oh is a peculiar transient supersoft X-ray source without a mass ejection signature in the field of the Small Magellanic Cloud. Maccarone et al. (2019) concluded that ASASSN-16oh is the first dwarf nova with supersoft X-ray that originated from an equatorial accretion belt on a white dwarf (WD). Hillman et al. (2019) proposed a thermonuclear runaway model that both the X-rays and V /I photons are emitted from the hot WD. We calculated the same parameter models as Hillman et al.'s and found that they manipulated on/off the mass-accretion, and their best fit V light curves are 6 mag fainter, and decay about 10 times slower, than that of ASASSN-16oh. We propose a nova model induced by a high rate of mass-accretion during a dwarf nova outburst, i.e., the X-rays originate from the surface of the hydrogen-burning WD whereas the V /I photons are from the irradiated disk. Our model explains the main observational properties of ASASSN-16oh. We also obtained thermonuclear runaway models with no mass ejection for a wide range of parameters of the WD mass and massaccretion rates including both natural and forced novae in low-metal environments of Z = 0.001 and Z = 0.0001. They are a new type of periodic supersoft X-ray sources with no mass ejection, and also a bright transient in V /I bands if they have a large disk. We suggest that such objects are candidates of Type Ia supernova progenitors because its mass is increasing at a very high efficiency (∼ 100%).

Theoretical Light-curve Models of the Symbiotic Nova CN Cha—Optical Flat Peak for 3 Yr

“…Chen et al (2019) presented a hydrogen shell flash calculation and obtained the ignition mass of M ig = 3.3 × 10 −5 M ☉ (the recurrence time P rec = 2500 yr) for a 0.6 M ☉ WD with Z = 10 −4 and CO-rich accretion of M = ´-☉ yr −1 . This ignition mass is close to that of model A, which suggests that the mass accretion rate of CN Cha is around 10 −8 M ☉ yr −1 .Chen et al (2019) also obtained the ignition mass of M ig = 2.2 × 10 −4 M ☉ (P rec = 22,000 yr) for a 0.6 M ☉ WD with no CO enrichment, Z = 10 −4 , and M = ´-☉ yr −1 Kato et al (2020). obtained the ignition mass as M ig = 2.0 × 10 −4 M ☉ (P rec = 10,000 yr) for a 0.6 M ☉ WD with Z = 0…”

“…The photospheric luminosity at the flat peak is brighter for more massive WDs, so that the (m − M) V becomes larger with increasing M WD . The horizontal solid/dotted blue lines indicate (m − M) V = 12.9 ± 0.3, calculated from a Gaia eDR3 distance, d 3.05 0.17 For models A, B, and C, we adopt a WD (radius) in a thermal balance with a relatively large mass accretion rate of several ×10 −8 M ☉ yr −1 (Kato et al 2020). For model D, we take a smaller WD radius for a colder WD for comparison, and increase the heavy-element enrichment to Z = 0.01.…”