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Close binary progenitors and ejected donor remnants of supernovae type Ia

Periodic Reporting for period 1 - SNBinaries (Close binary progenitors and ejected donor remnants of supernovae type Ia)

Reporting period: 2015-11-01 to 2017-10-31

Observations in the last decades indicate that our universe is not only expanding, but that this expansion accelerates. To explain this expansion, cosmologists introduced the concept of dark energy. The properties of this dark energy are basically unknown, despite the fact that it largely dominates the energy budget of the Universe. Deriving those properties is therefore one of the key unsolved problems in modern astronomy. The analysis of supernovae type Ia (SN Ia) is widely used as a tool to achieve that goal. SN Ia are very bright stellar explosions. The characteristic decay of their apparent brightness has been found to be related to their absolute luminosities. If the absolute luminosity of a source is known, its distance can be determined. Astronomers call such rather rare objects standard candles. SN Ia can be used to measure the largest distances in the universe. However, the progenitors of SN Ia explosions are still unknown and this might affect the applicability of SN Ia as standard candles. To investigate this it is not only necessary to unambiguously identify the progenitor population, but also to characterize its fundamental properties as detailed as possible.

The observed properties of SN Ia are best explained by the explosion of a white dwarf. White dwarfs are the end stages of stellar evolution for most stars. Extreme conditions are necessary to trigger the explosion of a white dwarf, which can only be reached, when matter is transferred by a companion star. Recently, close, eclipsing binaries consisting of white dwarfs and compact helium stars (hot subdwarfs, sdO/B) have been identified as important progenitor candidates. The detonation of accreted helium at the surface of the white dwarf is expected to trigger the SN Ia explosion. Because those binaries stars have very close orbits, the helium star companions are ejected after the SN Ia explosion with the most extreme velocities known in our Galaxy. This so-called helium double-detonation scenario therefore provides the unique opportunity to study both the progenitor sample and the sample of the ejected companions in detail. During this Marie Curie fellowship we want to use public data of time-domain surveys to identify the progenitors and ejected companions. Based on photometric and spectroscopic analyses, we want to characterise representative samples of them. These fundamental samples can be used in the future to reconstruct the formation and evolution of the progenitor systems and determine the effects when using SN Ia as cosmic distance indicators.
To search for hot subdwarf stars in close binary systems and single stars with very high velocities, we first compiled of an all-sky catalogue of essentially all known sdO/B stars consisting of more than 4400 stars.

Since close binary stars show characteristic variability in their light curves caused by eclipses and tidal effect, we visually inspected light curve data from the following surveys aimed to find exoplanets or astrophysical transients such as supernovae: Catalina Real Time Survey (completed), Palomar Transient Factory (completed in collaboration with T. Kupfer, Caltech), SuperWASP (ongoing), GALEX gPhoton (ongoing in collaboration with D. Wilson, Warwick). In this way we discovered about 20 new sdBs with low-mass stellar companions, several pulsating sdBs and one short-period sdB+WD binary. However, we found that systematics in the light curves limit the sensitivity to discover sdB+WD systems with shallow variations. Especially the systematics in the gPhoton light curves must be examined further.

To find high velocity we cross-matched the sdO/B catalogue with proper motion catalogues and found 65 hot subdwarfs with high velocities between >300-900 km/s. To obtain spectra of the best candidates we prepared proposals for observing time at the Gemini, William Herschel, SOAR and VLT telescopes.

Overview of the results:
* A new catalogue of hot subdwarf stars
* Discovery of a sample of variable hot subwdarf binaries, among them one potential candidate for being the progenitor of an SN Ia
* Discovery of a significant number of high velocity hot subdwarfs, which might be the ejected companions of SN Ia

Due to the early termination of the project (five months after the start of this Marie Curie action), these early results have not been published within its duration. Subsequently, however, we published an extended version of the hot subdwarf catalogue (Geier et al. 2017, A&A, accepted) and the close binary sdB+WD system we found (Kupfer et al. 2017, ApJ, 835, 131).
The catalogue of spectroscopically identified sdO/Bs contains twice as many objects as the previous version of 2006. With photometry from ultraviolet to the infrared, proper motions as well as spectroscopic parameters it is an excellent tool for cross-matching and data mining. It will become a particularly valuable resource as soon as the Gaia mission will provide distances to these stars. Although the search for compact SN Ia progenitor candidates turned out to be more difficult than expected (especially because of the systematics in the light curve data), we already found a very compact sdB+WD candidate showing ellipsoidal variations. The search for ejected companions on the other hand works much better than expected. From the 65 candidates discovered so far, 25 might be unbound to our Galaxy and there ejected (one is known so far).
The hypervelocity star US708 is propelled out of the Galaxy after a supernova explosion