Medium Resolution spectroscopy of the outbursting Be Star ASASSN-V J010932.93+614659.0
ATel #10713; I. A. Steele (Liverpool JMU)
on 6 Sep 2017; 14:40 UT
Credential Certification: Iain Steele (email@example.com)
Subjects: Optical, Star, Variables
ATel #10710 reports an ongoing optical outburst of the source ASASSN-V J010932.93+614659.0 (TYC 4030-604-1). They show a low resolution spectrum which identifies the source as a likely Be star.
I subsequently obtained a medium resolution (R=5000) spectra of the target on the night of 2017 Sept 05 using the FRODOSpec spectrograph of the Liverpool Telescope in the ranges 3900-5100 and 5900-8000 Angstroms. Strong, single peaked emission can be seen in the H-alpha line (EW = -5.1 Angstroms, not corrected for photospheric absorption). The H-beta line shows a weak emission component within the core of the photospheric line. There is no indication of He-I emission (e.g. at 6678 or 7065 Angstroms).
A Gaussian fit to the H-alpha emission line component has FWHM=4.2 Angstroms, corresponding to a velocity of 192 km/s.
Comparison of the blue spectrum with those of Be stars presented in Steele et al. (1999, A&AS, 137, 147), and particularly the ratios of the lines of He I 4471, Mg II 4481, He I 4009 and He I 4026, indicates a spectral classification of B5-B6 is appropriate. Based on the measured equivalent width of the H-gamma line (8.1 Angstroms) a luminosity classification V (i.e. main sequence) is preferred (e.g. Balona & Crampton 1974, MNRAS, 166, 203).
Gaussian fits to the FWHM of the He I 4387 and 4471 absorption lines give a mean projected rotational velocity v sin i = 160 +/- 10 km/s. The source is therefore a rapid rotator as is normal for Be stars.
The spectrum shows evidence of a diffuse interstellar bands, with EW (DIB 4430) = 1.1 Angstroms. From Herbig (1975, ApJ, 196, 129) this implies an extinction A_V = 1.3 magnitudes. Assuming the low state of the object is V=12.1 (from the ASASSN light curve prior to outburst) and using this extinction and assuming a spectral type of B6V, the estimated distance of the system is 2.1 kpc. This is consistent with the Gaia distance of 1.1 - 3.7 kpc.