CC BY
3ISSN 0236-2457 DOI:10.24412/0236-2457-2021-1649-1-4
ASTRONOMICHESKII TSIRKULYAR
Published by the Eurasian Astronomical Society and Sternberg Astronomical Institute
№ 1649, 2021 September 21
UNUSUAL FAST SPECTRAL VARIABILITY of 7 Cas A.F. Kholtygin1, M.A. Burlak2, and O.A. Tsiopa3
1 Saint-Petersburg University E-mail: afkholtygin@gmail.com 2Sternberg Astronomical Institute, Moscow University 3Main (Pulkovo) Astronomical observatory Received September 12, 2021, with corrections September 17, 2021
Abstract. An unusual fast variations in the spectra of Be star 7 Cas are analysed. Regular line profile variations (LPVs) with periods from 4 to 37 minutes are found.
Introduction
The most important problems of stellar spectroscopy is the study of the structure and mechanisms of the expanding atmospheres of massive stars formation. Such studies are based on an analysis of regular and stochastic variations in the line profiles in the stellar spectrum. In the present paper the observations of the B0.5IVpe star 7 Cas at the 1.25-m telescope in the Crimean station of Sternberg Astronomical Institute, Moscow University are analysed.
Observations and data reduction
B0.5IVpe star 7 Cas is the prototypical classical Be star and is recently best known for its variable extremely hard X-ray emission. That is the binary system with the primary mass of 15 MQ and low mass secondary 0.8 MQ [1].
The observations of 7 Cas were made with the 1.25-m telescope on the night of September 17/18, 2020. All spectra were obtained with an exposure time 2 s and the time resolution ~ 5 s including SSD reading-out time. Totally 1575 spectra were obtained. The duration of observation is 128.3 min. All our spectra were obtained in the range AA 4421 — 6860 A with a spectral resolution ~1000 and signal to noise ratio S/N~ 400.
The data reduction was made using the code CCDops1. One-dimensional spectra were obtained by summing the counts within a 40-pixel (79") aperture, at a mean FWHM of 26 pixels, with the subtraction of the sky background taken over a region of 60 — 120 pixels from the center of the stellar spectrum. The wavelength calibration was made using a Ne-Ar lamp. The spectra are normalized to the continuum. The normalization procedure is described by Kholtygin et al. [2].
1http://company7.com/library/sbig/sbwhtmls/ccdopsv5.html
Fast LPVs
Analyzing the difference profiles we will use the Doppler shifts V from the laboratory wavelength Ao of the line instead of the wavelength A. The difference line profile
d(V,t) = F(V,t) - F(V). (1)
Here F(V) is the mean flux in the line at the velocity V
_ 1 N
F(V) = N £ F(V,ti). (2)
i=0
where N is the total number of the analysed spectra, F(V, ti) is the continuum normalized line flux for the spectrum obtained at time ti. Dynamical spectra d(V, t) for Hg and Hel 5876 lines are given in Fig. 1.
Velocity (km/s) Velocity (km/s)
Figure 1: Dynamical spectra of lines Hg (top panel) and HeI5876 (bottom panel)
Inspecting this figure one can see the similarity of LPVs for Hg and Hel 5876 lines. Such similarity holds also for lines Ha and Hel 6678. Fast variations at the time scale of ~ 4 ^ 20 minutes are seen in Fig. 1.
Regular components of LPVs
For searching the periodic components of the line profile variations in the spectrum of y Cas the CLEAN method of Fourier analysis [3] was used. Fourier spectra of variations in the difference profiles of HeI5876, 6678, Hg and Ha lines (periodograms) are calculated. In Fig. 2 (left panel) the fit of the residual values d(V, t) for the line Hg at AV = -78 kms-1 is shown. An example of the full Fourier spectra for Ha line and low FAP (False Alarm Probability) level ~ 10-4 is given in Fig. 2 (right panel).
We obtain the upper limits of the regular component frequency errors and the errors of the corresponding periods in the Fourier spectrum using the expression Av ^ 1/T [4], where
Table 1: Periods of regular LPV's components
Comp. number 1 2 3 4 5
Period, min 4.02 ± 0.13 6.57 ± 0.34 10.35 ± 0.84 20.82 ± 3.38 37.11 ± 10.76
T = 128.3 minutes is the total duration of observations. The detected periods together with their errors are given in Table 1.
We have tabulated only those periods P for which the ratio P/AP > 3 where AP is the period's error. Additionally we revealed 3 components with periods from 50 to 116 minutes. Longer observations are needed to confirm their reality. Given short-time LPVs are firstly detected in spectra of 7 Cas was not known before but they are similar to those for bright OB stars [5,6].
0 20 40 60 80 100 120
Time (min)
-400 0 400
Velocity (km/s)
8000 7000 6000 5000 4000 3000 2000 1000
Figure 2: Fit (blue line) for Hg residuals at V = — 78kms 1 and FAP=10-4 (right panel)
(left panel) and Fourier spectra for H0
References
1. Smith M.A., Lopes de Oliveira R., Motch C., et al., A&A 540, A53 (2012).
2. Kholtygin A.F., Burlakova T.E., Fabrika S.N., Astron. Rep. 50, 887 (2006).
3. Roberts D.H., Lehar J., Dreher J.W., Astron. J. 93, 968 (1987).
4. Vityazev V.V., Analiz neravnomernyih ryadov (SPbSU Press, St. Petersburg, 2001, in Russian).
5. Kholtygin A.F., Batrakov A.A., Fabrika S.N., et al., Astroph. Bull., 74, 392 (2018).
6. Kholtygin A.F., Moiseeva A.V., Kurdoyakova M.S., et al., Astroph. Bull., 76, 185 (2021).
НЕОБЫЧНО БЫСТРАЯ СПЕКТРАЛЬНАЯ ПЕРЕМЕННОСТЬ y CAS
А.Ф. Холтыгин1, М.А. Бурлак2, и О.А. Циопа3
1 Санкт-Петербурский госуниверситет, E-mail: afkholtygin@gmail.com
2Государственный Астрономический Институт имени П.К. Штернберга МГУ
3Главная (Пулковская) Астрономическая обсерватория
Резюме. Проанализирована быстрая переменность профилей линий в спектре y Cas. Обнаружены регулярные вариации профилей с периодами от 4 до 37 минут.
