Measuring starspot temperature from line depth ratios

Abstract

Abstract.Gray and collaborators have recently demonstrated that line-depth ratios are a powerful tool for temperature discrimination, able to resolve differences ≤10 K. The method has been applied to detect temperature variations in the 5-15 K range due to activity cycles (e.g. Gray et al. 1996aGray et al. , 1996b or to rotation modulation produced by large surface features, called "star-patches", like that detected in ξ Boo A by Toner & Gray (1988). Cool starspots of a few tenths of the stellar surface produce bumps in a line profile, which migrate through the line profile allowing Doppler-imaging in fast rotating stars. In the hypothesis that in slowly-rotating stars the passage of dark spots produces modulation of the center line depth of different amount in lines of different sensitivity to temperature, we have made test observations on three active binaries of the RS CVn type. Based on observations made at the Catania Astrophysical Observatory at a resolution R = 14 000, we show that line-depth ratios can be effectively used to determine spot temperatures of active binary systems. Using, on average, ten line pairs, selected in the 6100-6300 Å wavelength range, with the help of observations of 30 main sequence and giant stars, we have derived a calibration relation of line-depth ratios (LDR) in an absolute temperature scale, taking into account the gravity effect in the calibration relation. Single LDRs converted to temperature through the calibration relations have led to clear rotational modulation of the average surface temperature with amplitudes of 177 K, 119 K, and 127 K for VY Ari, IM Peg and HK Lac, with average estimated errors of about 10 K. We show that the observed temperature variation amplitude allows us to define a minimum fractional spotted area coverage as a function of spot-photosphere temperature ratio. Adopting the maximum value of average temperature, determined from the LDRs, as that of the unspotted photosphere, we computed the average spot temperature corresponding to the minimum spot coverage. Although not univocally constrained, the temperature difference (∆T = T ph − T sp ) obtained for the three systems, ∆T = 890 K for VY Ari, ∆T = 750 K for IM Peg, and ∆T = 810 K for HK Lac, are in good agreement with values derived with other methods.

Line-depth-ratio temperatures for the close binary ν Octantis: new evidence supporting the conjectured circumstellar retrograde planet

“…The next four highest scatters are for ratios including the Fe II line 6247Å. This latter result is consistent with findings commented upon by Catalano et al (2002), whose two LDRs including this line (r4347 and r4647) had such high gravity-dependence that they could not be used for all of their calibration stars. Similarly, our ratio r5253 shows a very small scatter and Catalano et al illustrated the tiny gravity effect this ratio has by the near coincidence of their main-sequence and giant star calibration curves.…”

“…An estimate of the stage of evolution (corresponding to a gravity index) was calculated by taking the difference between the absolute magnitude and the star's zero-age main-sequence (ZAMS) magnitude at that temperature (see e.g. Catalano et al 2002). A cubic polynomial fit was made to the ZAMS values given in Allen (1991) and this locus was compared to the MV vs. T (B−V ) 0 values of nearly 2000 Hipparcos stars within 80 pc, again using Eq.…”

“…Gray & Johanson 1991;Gray 1994;Kovtyukh et al 2003), giant (e.g. Gray & Brown 2001;Kovtyukh et al 2006), and supergiant stars (Kovtyukh 2007;Pugh & Gray 2013) as well as active stars (Gray & Baliunas 1995;Padgett 1996;Catalano et al 2002) including the Sun's 11-year cycle (Gray & Livingston 1997). In turn, these temperatures can be used to monitor and assess starspots (e.g.…”

“…In turn, these temperatures can be used to monitor and assess starspots (e.g. active stars; Catalano et al 2002;O'Neal 2006;Biazzo et al 2007), pulsation cycles (e.g. of Cephieds: Kovtyukh &Gorlova 2000, andAntares A: Pugh &Gray 2013), and hence to help support claims for the non-existence or existence of exoplanets (e.g.…”

Evidence for a Planetary Companion around a Nearby Young Star

“…The relative rms of the Sindex variation is 4.5% of the mean value. In addition, we also measured the EW variation of the Ha line and the variation T eff by using the line-ratio technique (e.g., Catalano et al 2002) to search for the stellar activity. The EW measurements of the Ha line give a value of m .…”