Looking for pulsations in HgMn stars through CoRoT lightcurves

Abstract

Context. HgMn chemically peculiar stars are among the quietest stars of the main-sequence. However, according to theoretical predictions, these stars could have pulsations related to the high overabundance of iron peak elements, which are produced by atomic diffusion in upper layers. Such pulsations have never been detected from ground-based observations. Aims. Our aim is to search for signatures of pulsation in HgMn stars using the high quality lightcurves provided by the CoRoT satellite. Methods. We identified three faint stars (V > 12), from a VLT-GIRAFFE multiobject spectrographic survey in a field planned for observation by CoRoT. They present the typical characteristics of HgMn stars. The three stars were observed by the CoRoT satellite during the long run (131 days) which started on 24 October 2007, with the exoplanets CCDs (Additional Programme). In the present work, we present the analysis of the ground-based spectra of these three stars and of the corresponding CoRoT lightcurves. Results. Two of these three HgMn candidates show low amplitude (less than 1.6 mmag) periodic variations (4.3 and 2.53 days respectively, with harmonics) which are compatible with periods predicted by theoretical models.

Dynamical evolution of titanium, strontium, and yttrium spots on the surface of the HgMn star HD 11753

“…Searches for variability in this group of stars have been made mostly photometrically in the past, but without any success. Very recently, Alecian et al (2009) discovered low amplitude (less than 1.6 mmag) Based on observations obtained with the CORALIE Echelle Spectrograph on the 1.2-m Euler Swiss telescope, situated at La Silla, Chile.…”

A search for pulsations in the HgMn star HD 45975 with CoRoT photometry and ground-based spectroscopy

“…From the observational side, however, single HgMn stars do not show any evidence of large photometric changes (Adelman 1998). This is rather unexpected considering the theoretical predictions, and it suggests that the models are still too crude and do not capture all the complexity of these objects, as indeed cautioned by Turcotte & Richard (2003, 2005; Turcotte (2005); and Alecian et al (2009).…”

“…The models of Turcotte & Richard (2003) predict that HgMn stars should present a large iron opacity bump at about 2 × 10 5 K that would be very favourable to driving pulsations through the κ mechanism. Calculations by Alecian et al (2009) also suggest the excitation of non-radial g modes as found in slowly pulsating B stars (SPBs). From the observational side, however, single HgMn stars do not show any evidence of large photometric changes (Adelman 1998).…”

“…Much tighter observational constraints have recently been provided by ultra-high precision photometric observations with the CoRoT satellite (Baglin et al 2009), which revealed for the first time periodic (with periods of 2.53 and 4.3 d) and lowamplitude (less than 1.6 mmag) changes compatible with pulsational instabilities in two faint (V > 12 mag) HgMn stars in the exoplanet fields (Alecian et al 2009). As discussed below, the existence of spatially-extended abundance patches has been established from spectroscopy in a few HgMn stars (e.g., Adelman et al 2002).…”

“…Since that pioneering work, high-altitude accumulations have often been considered and discussed in papers devoted to modelling atomic diffusion in atmospheres. Recently, Alecian (2012) has proposed that abundance spots in some HgMn stars could be due to atomic diffusion in a weak (the diffusion velocity was computed for 100 Gauss in Alecian 2013, suggesting a cloud location at log τ = −4) 1 and faintly organised magnetic field in their atmosphere, i.e., multipolar, varying on small spatial scales, and difficult to detect (see also Alecian et al 2009). Besides the possible effect of weak magnetic fields on the formation of high-altitude spots of some elements, the buildup of stratifications in the upper atmosphere is very complex since this process is non-linear, time-dependent, and possibly unstable (see Alecian et al 2011).…”