Mode identification from monochromatic amplitude and phase variations for the rapidly pulsating subdwarf B star EC 20338−1925

Abstract

We obtain time-series spectrophotometry observations at the VLT with the aim of partially identifying the dominant oscillation modes in the rapidly pulsating subdwarf B star EC 20338−1925 on the basis of monochromatic amplitude and phase variations. From the data gathered, we detect four previously known pulsations with periods near 147, 168, 126 and 140 s and amplitudes between 0.2 and 2.3 % of the star's mean brightness. We also determine the atmospheric parameters of EC 20338−1925 by fitting our non-LTE model atmospheres to an averaged combined spectrum. The inferred parameters are T eff = 34 153 ± 94 K, log g = 5.966 ± 0.017 and log N(He)/N(H) = −1.642 ± 0.022, where the uncertainty estimates quoted refer to the formal fitting errors. Finally, we calculate the observed monochromatic amplitudes and phases for the periodicities extracted using least-squares fitting to the light curves obtained for each wavelength bin. These observed quantities are then compared to the corresponding theoretical values computed on the basis of dedicated model atmosphere codes and also taking into account non-adiabatic effects. We find that the quality of the data is sufficient to identify the dominant pulsation at 146.9 s as a radial mode, while two of the lower amplitude periodicities must be low-degree modes with = 0−2. This is the first time that monochromatic amplitudes and phases have been used for mode identification in a subdwarf B star, and the results are highly encouraging.

Mode identification based on time-series spectrophotometry for the bright rapid sdB pulsator EC 01541−1409

“…This is always dependent on the availability of very high S/N observational data that are extremely challenging to gather and require medium to large telescopes. On the basis of the theoretical framework developed by Randall et al (2005), we have so far achieved formal and unambiguous identifications for the following pulsations: 1) Randall et al (2005) used the high-quality WHT/ULTRACAM u g r multicolour photometry gathered on KPD 2109+4401 (Jeffery et al 2004) to formally identify the largest amplitude 182.4 s mode detected in that pulsator at the time as a radial mode; 2) Charpinet et al (2008b) presented an analysis of the exceptionally high S/N UBV photometry obtained with the CFHT/LAPOUNE combination on the bright target Balloon 090100001 to identify the dominant mode as a radial pulsation, and eight other modes with values of = 1 and 2; 3) Silvotti et al (2010) used high S/N WHT/ULTRACAM photometry to associate the dominant 349.5 s mode detected in HS 2201+2610 with a radial mode and the second-highest amplitude mode at 354.1 s with an = 1 mode; 4) Randall et al (2010) used time-resolved spectrophotometry gathered with the VLT/FORS combination to identify the dominant mode at 146.9 s as a radial mode in the relatively faint star EC 20338−1925, at the same time establishing the feasibility of the monochromatic approach; and 5) Using the same observational set-up, we demonstrated in this paper that the P 1 mode in EC 01541−1409 is a radial mode, while the much lower amplitude P 2 mode is an = 2 pulsation. Not surprisingly perhaps, the expected amplitude hierarchy seems to be well followed in pulsating sdB stars, and dominant pulsations generally correspond to radial modes.…”

“…Fig. 7 from Randall et al 2010). Both the amplitude and phase plots clearly trace the H/He spectral lines as well as the continuum behaviour as a function of wavelength.…”

“…This kind of amplitude variation is well-documented for many sdBV r stars and is thought to be inherent to the opacity mechanism driving the pulsations rather than the beating of unresolved multiplets in at least some cases (e.g. Randall et al 2010). However, we currently lack the non-linear pulsation theory necessary to model the intrinsic amplitudes of different modes, and are therefore unable to quantitatively interpret absolute amplitudes or their variation.…”

Asteroseismology of hot B subdwarf stars

“…These include surveys to find the pulsators [4,9,10], dedicated groundbased white light photometric campaigns to provide key data for asteroseismology [11,12] (see Fig. 2), attempts to identify modes through multicolor photometry [13][14][15] and time resolved spectroscopy [16,17], and most recently ultra high precision photometry from space with CoRoT [18] and Kepler [19,20] (see Fig. 2).…”

Identifying and characterising the population of hot sub-luminous stars with multi-colour MeerLICHT data

“…eclipses, reflection effects, and ellipsoidal modulation), sdOB stars have been observed to exhibit both pressure (p) and gravity (g) mode oscillations, with amplitudes ranging from micro-to milli-magnitudes (Kilkenny et al 1997;Charpinet et al 1997;Green et al 2003;Fontaine et al 2003). The presence of pulsations has enabled asteroseismology to characterise the rotation rates of these stars, determine their internal (chemical) structures, and estimate the mass of the thin hydrogen envelope (Telting & Østensen 2006;Hu et al 2007;Vučković et al 2009;Randall et al 2010;Van Grootel et al 2010;Charpinet et al 2011;Pablo et al 2012;Østensen et al 2014;Zong et al 2016;Ghasemi et al 2017;Lynas-Gray 2021).…”