R Coronae Borealis at the 2003 light minimum

Abstract

A set of five high-resolution optical spectra of R CrB obtained in 2003 March is discussed. At the time of the first spectrum (March 8), the star was at V = 12.6, a decline of more than six magnitudes. By March 31, the date of the last observation, the star at V = 9.3 was on the recovery to maximum light (V = 6). The 2003 spectra are compared with the extensive collection of spectra from the 1995–1996 minimum presented previously. Spectroscopic features common to the two minima include the familiar ones also seen in spectra of other R Coronae Borealis stars (RCBs) in decline: sharp emission lines of neutral and singly ionized atoms, broad emission lines including He i, [N ii] 6583 Å, Na D and Ca ii H & K lines, and blueshifted absorption lines of Na D, and K i resonance lines. Prominent differences between the 2003 and 1995–1996 spectra are seen. The broad Na D and Ca H & K lines in 2003 and 1995–1996 are centred approximately on the mean stellar velocity. The 2003 profiles are fit by a single Gaussian, but in 1995–1996 two Gaussians separated by about 200 km s-1 were required. However, the He i broad emission lines are fit by a single Gaussian at all times; the emitting He and Na–Ca atoms are probably not colocated. The C2 Phillips 2–0 lines were detected as sharp absorption lines and the C2 Swan band lines as sharp emission lines in 2003, but in 1995–1996 the Swan band emission lines were broad and the Phillips lines were undetected. The 2003 spectra show C i sharp emission lines at minimum light with a velocity changing in 5 d by about 20 km s-1 when the velocity of 'metal' sharp lines is unchanged; the C i emission may arise from shock-heated gas. Reexamination of spectra obtained at maximum light in 1995 shows extended blue wings to strong lines with the extension dependent on a line's lower excitation potential; this is the signature of a stellar wind, also revealed by published observations of the He i 10830 Å line at maximum light. Changes in the cores of the resonance lines of Al i and Na D (variable blueshifts) and the Ca ii infrared (IR) lines (variable blueshifts and redshifts) suggest complex flow patterns near the photosphere. The spectroscopic differences at the two mimima show the importance of continued scrutiny of the declines of R CrB (and other RCBs). Thorough understanding of the outer atmosphere and circumstellar regions of R CrB will require such continued scrutiny.