Model atmosphere analysis of the extreme DQ white dwarf GSC2U J131147.2+292348

Abstract

Abstract.A new model atmosphere analysis for the peculiar DQ white dwarf discovered by Carollo et al. (2002) is presented. The effective temperature and carbon abundance have been estimated by fitting both the photometric data (UB J VR F I N JHK) and a low resolution spectrum (3500 < λ < 7500 Å) with a new model grid for helium-rich white dwarfs with traces of carbon (DQ stars). We estimate T eff 5120 ± 200 K and log[C/He] −5.8 ± 0.5, which make GSC2U J131147.2+292348 the coolest DQ star ever observed. This result indicates that the hypothetical transition from C 2 to C 2 H molecules around T eff = 6000 K, which was inferred to explain the absence of DQ stars at lower temperatures, needs to be reconsidered.

Detailed Spectroscopic and Photometric Analysis of DQ White Dwarfs

“…We have validated our model atmosphere calculations by comparing our results with those of Provencal et al (2002) and Carollo et al (2003), which both rely on the DQ models of D.…”

“…The most obvious explanation proposed by BRL97 to account for the sudden drop in the number of DQ stars below T eA $ 6500 K is that DQ stars turn into C 2 H stars as they cool off. This scenario has recently been challenged by Carollo et al (2003), who reported the discovery of a DQ star with very strong C 2 Swan bands (see also Carollo et al 2002) at an estimated effective temperature of $5120 K, well below the empirical cutoff alluded to by BRL97. Taken at face value, this result would suggest that the cutoff is simply a selection effect and that the DQ phenomenon persists at low effective temperatures.…”

The GSC-II-based survey of ancient cool white dwarfs

“…7 shows the coolest (5120 K) carbon rich WD known to date: GSC2U J131147.2+292348 described in Carollo et al (2002Carollo et al ( , 2003. Another similar DQ was recently discovered in the SDSS (Kleinman et al 2004); its modelling by Dufour et al (2005) yields a temperature of 5140 K, which increases to two the DQs with temperature below the expected cutoff of 6000 K.…”

“…Monet et al (1992) and Bergeron et al (1997). b Peculiar DQ with extremely strong C 2 bands (Carollo et al 2002(Carollo et al , 2003. c Heavily blanketed DZ (G 165-7) (Wehrse & Liebert 1980).…”

Spectropolarimetric observations of cool DQ white dwarfs

“…On the other hand, Koester & Knist (2006) modelled it as peculiar DQ WD with a temperature of 5846 K based on photometry. There are at least two other DQ WDs with similar spectra: GSC2U J131147.2+292348 (Carollo et al 2003) and SDSS J090208.40+20104 (Limoges et al 2013). We think that some attention needs to be given to these stars in the future because they might give new insight into the evolution of hot DQs.…”