High-resolution spectroscopy of V854 Cen in decline - absorption and emission lines of C2molecules

Abstract

ABSTRACT High-resolution optical spectra of the R Coronae Borealis (RCB) star V854Centauri inthe early stages of a decline show, in addition to the features reported for other RCBsin decline, narrow absorption lines from the C 2 Phillips system. The low rotationaltemperature, T rot = 1150K, of the C 2 ground electronic state suggests the cold gas isassociated with the developing shroud of carbon dust. These absorption lines were notseen at a fainter magnitude on the rise from minimum light nor at maximum light.This is the first detection of cold gas around a RCB star.Key words: Star: individual: V854Cen: variables: other 1 INTRODUCTIONR Coronae Borealis stars are H-poor F-G type supergiantsthat decline in brightness unpredictably by up to 8 mag-nitudes and remain below their normal brightness for sev-eral weeks to months. It is generally accepted that thesedeclines are due to formation of a cloud of carbon soot thatobscures the stellar photosphere. Unanswered questions re-main: ‘What triggers cloud formation?’ and ‘Where doesthe soot form?’ High-resolution spectroscopic monitoring ofRCBs from maximum light into decline will likely be neces-sary to refine schematic ideas into answers that are accordedwidespread acceptance. We report the first detection of coolgas (T ≃ 1100K) during the early decline of a RCB starand, hence, evidence for a site of soot formation. Cold dustis, of course, known around RCBs through detection of aninfrared excess.The RCB in question is V854Cen, which at maximumlight is the third brightest RCB variable after RCrB andRYSgr. V854Cen is presently the most variable of all Galac-tic RCBs. Despite the combination of favorable apparentmagnitude and propensity to fade, there is a dearth of high-quality spectroscopic observations of this star in decline.The sole report of a high-resolution optical spectrum cov-ering a broad bandpass in a deep decline is that by Rao &Lambert (1993) taken when the star had faded by about8 mag. Low resolution spectra are described by Kilkenny &Marang (1989) and spectropolarimetric observations are dis-cussed by Whitney et al. (1992). Spectra at high-resolutionat maximum light were used by Asplund et al (1998) fortheir abundance analysis that confirmed that V854Cen hasa somewhat unusual composition among the RCBs for whichabundance anomalies are a sine qua non. In particular,V854Cen, although hydrogen-poor relative to normal stars,is the most hydrogen-rich RCB by a clear margin. Despitelimited temporal coverage, our new spectra of V854Cen indecline provide a novel result - the detection of cold C