Very Short-Term lpv in the Be Star  Aqr

Presented at the meeting of the Working Group on Active B Stars during the 25th IAU General Assembly in Sydney, Australia on 2003 July 16

G. J. Peters¹ and D. R. Gies²

¹Space Sciences Center/Dept. of Physics & Astronomy, University of Southern California, Los Angeles, CA 90089-1341, USA
email: gjpeters@mucen.usc.edu
²CHARA/Dept. of Physics & Astronomy, Georgia State University, Atlanta, GA 30303-3083, USA
email: gies@chara.gsu.edu

Received: 2004 July 21; Accepted: 2004 July 26.

Almost 140 years have passed since Fr. Secchi discovered H emission in  Cas, but the physics that underlies the Be phenomenon still remains a subject for debate. The photospheric activity that presumably leads to the mass loss has been elusive and appears to often be obscured by the star's optically thick circumstellar disk. Therefore, it seems reasonable to study the spectroscopic behavior of the Be star during a non-emission phase to gain important insight into the cause for the disk buildup. An opportunity to examine the photosphere of such a Be star was provided in the mid-1990s when  Aqr lost its CS disk after about 5 decades of emission (McLaughlin 1962, Bjorkman et al. 2002).

 Aqr was observed in the 6525-6690 Å region with the Coudé Feed Telescope at the Kitt Peak National Observatory in 1999 November, 2000 October/November 3, and 2001 January. A total of 55 images were obtained. The observing setup produced a spectral resolution of 0.103 Å pixel^-1. The exposure time for each image was 15^m with a typical recycle time of 10^s between spectra. For most of the observations the CCD was operated in an automatic mode in which the data were taken, read out, and the next exposure started without delay. Observation sets spanned 1.5-3.0 hr. The spectra were flat-fielded, wavelength calibrated, and normalized to the local continuum using the IRAF software. The S/N ratio varied from 200-500 pixel^-1, with typical values in the 300-350 range.

Obvious short-term lpv was seen in 1999 November and 2001 January, but the variability was quite conspicuous in 2000 November. The lpv can be best described as a series of rapidly traveling absorption bumps with a period ~2^hr. Similar behavior in 1999 and 2001 was also reported by Rivinius et al. (2003). The nature of the variability is illustrated in the grayscale plot shown in Fig. 1. The traveling bumps in HeI 6678 Å, seen on 2000 November 1 are clearly seen. All other photospheric lines covered in our CCD images, including H and the CII doublet on its red wing, revealed the same behavior.

Figure 1. The lpv in HeI 6678 Å, on 2000 November 1. The flux deviation from the mean profile (lower panel) is displayed in gray tones. Dark regions pertain to greater flux absorption. Observation time increases downward and the arrows to the right show the locations of the individual spectra.

The line profile data were analyzed using the techniques and Fourier Transform software described in Gies & Kullavanijaya (1988). Three time groups of spectra were considered: 1999 November 28, 2000 October 30 to November 1, and 2001 January 5-8. A strong frequency peak of 12.8 c/d (P = 1.88 hr) was seen in all three datasets, each of which was modeled with an NRP p-mode of =-m=5±1. It is highly unlikely that the lpv occurs in the disk. Keplerian periods for material in an inner disk would be ˜1 day or longer. The traveling bumps, which were more visible in late 2000, appear instead to originate in the photosphere of the star. The absorption bumps in H show evidence of significant Stark broadening at their site of formation, which suggests a photospheric origin. The activity seen in  Aqr is very different from the typical behavior that was observed in  Cen during its non-emission epoch (Rivinius et al. 1998a,b, Peters 1986, 1998). Spectacular transients were not observed in  Aqr as in  Cen, nor was a low-order (=2), half-day, g-mode. The high-order NRP in  Aqr is very regular and more reminiscent of what one sees in a Bn star. The extent to which the star's binarity (Bjorkman et al. 2002) influences the photospheric activity is unknown. A more complete account of the observations and analysis can be found in a paper that will be submitted to the Astrophysical Journal.

References:

Bjorkman, K.S., Miroshnichenko, A.S., McDavid, D., & Pogrosheva, T.M. 2002, ApJ, 573, 812

Gies, D.R, & Kullavanijaya, A. 1988, ApJ, 326, 813

McLaughlin, D.B. 1962, ApJS, 7, 65

Peters, G.J 1986, ApJ, 301, L61

Peters, G.J 1998, ApJ, 502, L59

Rivinius, Th., Baade, D., & Stefl, S., Stahl, O., Wolf, B., Kaufer, A. 1998a, A&A, 333, 125

Rivinius, Th., Baade, D., & Stefl, S., Stahl, O., Wolf, B., Kaufer, A. 1998b, A&A, 336, 177

Rivinius, Th., Baade, D., & Stefl, S. 2003, A&A, 411, 229

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