Short time-scale variability in the spectrum of O-B hot stars

S.M.Pokhvala
Main Astronomical Observatory of NAS of Ukraine, Akademika Zabolotnoho str., 27, 03680 Kyiv, Ukraine

We reported the first results of observations of short time-scale variability in the hydrogen Balmer lines and HeI lines  in the hot O-B stars (η UMa, ζ Ori). Spectral observations were carried out with low-resolution slitless spectrograph (R ~ 200) installed on the 60 cm Carl Zeiss telescope in the Andrushivka Observatory, Ukraine. Spectra were obtained with a time resolution in the sub-second and second range. It has been found that the hot stars shows rapid variations in the hydrogen lines Hβ, Hγ, Hδ, Hε, Hη and helium lines HeI(4472Å), HeI(5016Å) HeI(5876Å) as well as variations in the atmospheric oxygen lines. This can be interpreted that their pulsations are non-radial pulsations and strong stellar wind.

INTRODACTION

Supergiants are evolved high-mass stars, larger and more luminous than main-sequence stars. O class and early B class stars with initial masses around 10-100 M☉ evolve away from the main sequence in just a few million years as their hydrogen is consumed and heavy elements start to appear near the surface of the star. The hot O-B stars have powerful stellar winds. In the UV range they show the profiles of P Cygni line. The hot O-B stars show the rate of expansion ~ 2000 km/s and weight loss to ~ 10-6 M⊙/year. The hottest stars can have the wind speed up to 4000 km/s. The winds coming from the hot O-B stars have numerous resonance lines.

In 1968, a 12-inch telescope and low-resolution (R~600) ultraviolet spectrophotometer was flown on an Aerobee 150 rocket to record the absolute ultraviolet spectra (Ly-α) of bright stars in the region of the constellation Orion[2]. Variation of the general slope of the spectra of the earlier-type stars with luminosity class can be observed.

In 2015, author reported first results of the detection of high-frequency variations in the hydrogen Balmer lines in the hot star η UMa of spectral class B3V[3].

OBSERVATIONS

The first spectral observations of η UMa were carried in July 2014 in the Andrushivka Astronomical Observatory. The goal of the observations was to obtain the spectra of hot stars (the divisors) in order to calibrate the spectra of the program chromospherically active stars.

Zeiss-600 in Andrushivka Observatory

However, while processing the observations, was found that η UMa shows the fast variation in the hydrogen lines[3]. Such variations are mainly typical for late-type stars with chromospheric activity. To investigate the spectral variability of η UMa 200 spectra were obtained with an exposure of 0.1 seconds and a time resolution of 1.96 seconds.

The second part of the observations was conducted at the Cassegrain telescope Zeiss-600 in Andrushivka Observatory from July till October 2016 with slitless grating spectrograph. Also, was used small aperture telescopes (6-8 inches) with slitless spectrograph (R~100).

Filter wheel with SA200

MAK-10 with Atik 314+

After detection variations in the η UMa was started observation company for some other O-B hot stars. In period from July 2016 till April 2017 was conducted 5 hot stars, such as ζ Ori, δ Ori, 10 Lac, 68 Cyg, ζ Oph. Fig. 1 shows recorded spectra of η UMa.

Figure. 1. Low-resolution slitless spectra of η UMa

Figure.2. Absorption spectrum of η UMa

To verify the presence of variability it is necessary to show that such a variability is absent in a comparison star. For this purpose, was used the comparison star 77 Cyg of spectral type A0V, V = 5.73.

Figure.3. The comparison of the variability in the spectra of low resolution of η UMa and 77 Cyg (sp. A0).

The Hydrogen lines show the bimodal structure of a spectral line (Fig.4, Fig.5). The Oxygen line (the Earth's atmosphere) doesn't show it. This proves the reality of the bimodal structure of a spectral Balmer lines. This bimodal structure in line profiles can be interpreted as a non-radial pulsations.

Figure.4. bimodal structure of the Balmer spectral lines

Figure.5. The Oxygen lines (the Earth's atmosphere)

Fig.6 shows recorded spectra of ζ Ori. Was used more 350 spectra for observation line of 25 min.

Fig.7 shows the relative variations in spectra (standard deviation divided by mean) in the Balmer and He I spectral lines as well as absorbing molecules of O2 and H2O.

Figure. 6. A series of low-resolution spectra of ζ Ori and absorption spectrum with spectral resolution of 16Å

Figure. 7. The relative variations in spectra ζ Ori

For deep investigations was used Alan method for detection of variability. It is expressed mathematically as

 

The Allan power spectrum (Fig.8) (covariation divided by mean) shows clearly variations in the Balmer and He I spectral lines. The active spectral lines show the maximum variability. However, the passive lines do not show the activity.

Fig.8. Alan Variations in Balmer lines and HeI lines in spectrum of ζ Ori.

In 1974 Peter  S. Conty presented the wind velocities of ζ Ori [4]. He found that the ζ Ori have strong P Cygni profiles in λ4686 and λ5876.Velocity of wind was calculated the 1400 km/s for ζ Ori.

CONCLUSIONS

The variability in the spectra of the O-B hot stars with short-scale time resolution gives to conclude that the hot OB stars have the activity in the Balmer lines and HeI lines. This can be interpreted that their pulsations are non-radial, that is, they vary in shape rather than volume; different parts of the star are expanding and contracting simultaneously. Also, it can be interpreted as the fast and strong stellar wind.

First results of investigation variability in ζ Ori show active variability in Balmer lines and HeI lines.

Last results confirm that hot star η UMa has the activity and it gives the possibility to classify this as Slowly pulsating B-type star (SPB) contrary to the previously known stability of the star.

REFERENCES

1. Georges Meynet; Cyril Georgy; Raphael Hirschi; Andre Maeder; Phil Massey; Norbert Przybilla, eds. "Red Supergiants, Luminous Blue Variables and Wolf-Rayet stars: The single massive star perspective". Societe Royale des Sciences de Liege, Bulletin, (Proceedings of the 39th Liege Astrophysical Colloquium. Liège (published 2011). 80 (39): 266–278.

2. Fank E. Stuart. Far-ultraviolet spectrophotometry of bright stars in Orion. The Astophysical Journal, vol.157. september 1969.

3. S.M. Pokhvala. High-frequency variations of hydrogen spectral lines in the B3V star η UMa. Advances in Astronomy and Space Physics, 5, 21-23, 2015.

4. Peter  S. Conty. Spectroscopic observations of O-types stars. The Astronomical Journal, 193, 1974, October 1.