Researchers at Yunnan Observatories, Chinese Academy of Sciences (CAS) recently performed a detailed asteroseismic analysis on an extremely low-mass white dwarf (WD), which exhibits suspected pressure modes (p-mode) pulsations. It not only detects the abundance profiles inside the WD, but also provides theoretical support for the reality of the p-mode. The research results were recently published in The Astrophysical Journal.
Some unstable mechanisms in the interior of stars can excite pulsations. Waves propagate in the interior of stars and form standing waves, which show periodic vibrations on the surface of stars. By observing and analyzing the frequencies of stellar pulsations, we can understand the physical process occurring inside the stars and determine the composition and structure of the stars. This method is called asteroseismology. There are two types of pulsation waves in the interior of stars: one is the p-mode that similar to the sound wave, which has a high frequency; the other is the g-mode (gravity mode) that similar to the internal gravity wave, which has a low frequency.
The periods of p-mode pulsations are usually in the order of seconds or less on WDs, due to large mean density and high surface gravity of WDs. This makes it a challenge to detect these pulsations. However, for an extremely low-mass WD (with a mass less than 0.2 solar mass) the periods of p-mode pulsations can be as long as 100 seconds and has an observable amplitude, which would be observed.
SDSS J111215.82+111745.0 is the second pulsating extremely low-mass WD discovered. Two shortperiod pulsations, 107.56 and 134.275 s, were detected on this star, which would be the first observed p-mode pulsations observed on a WD.
In this work, the researchers analyzed the whole set of seven periods (two p-modes and five g-modes) observed on SDSS J111215.82+111745.0. They perform asteroseismic modeling for this star, in which the H chemical profile is taken as a variable. The stellar parameters M = 0.1650 ± 0.0137 solar mass and Teff = 9750 ± 560 K are determined from the best-fit model, and the H/He chemical profiles are also defined. The two suspected p-modes are also well represented in the best-fit model, and both the stellar parameters and the pulsation frequencies are in good agreement with the values derived from spectroscopy. It provides theoretical support for the reality of these two p-modes.
This research is supported by the National Key R&D Program of China, by the National Natural Science Foundation of China (NSFC), and by the B-type Strategic Priority Program funded by the CAS.
Contact:
SU Jie
Yunnan Observatories, CAS
Email: sujie@ynao.ac.cn
