Peremennye Zvezdy (Variable Stars) 42, No. 9, 2022 Received 24 October; accepted 2 November.	Article in PDF
	DOI: 10.24412/2221-0474-2022-42-51-59

CCD Photometry of Young Stars AS 205 (V866 Sco), V1082 Cyg, and AS 310

Ulvi Valiyev¹, Sabahattin Alishov², Nariman Ismailov ²

1. Batabat Astrophysical Observatory of Nakhichevan Branch of Azerbaijan National Academy of Sciences, Azerbaijan

2. N. Tusi Shamakhy Astrophysical Observatory of Azerbaijan National Academy of Sciences, Shamakhy region, Azerbaijan

1. Introduction

In most cases, the brightness of classical T Tauri stars (CTTS) varies in a complex manner (Parenago, 1954; Herbst et al., 1994; Ismailov, 2005; Grankin, 2016). It is possible that irregular disk accretion in stars of this type causes an inhomogeneity of the stellar surface, so that the star's axial rotation can provide quasi-cyclic light variations. However, in some cases, the amplitude of light variation exceeds 1 mag, and the time scale of observed variations can be quite large. Therefore, it is desirable to obtain as many photometric observations of such objects as possible in order to conduct research in each specific case. In this work, the results of photometric observations of two CTTS and one Herbig AeBe (HAeBe) star, acquired in 2016-2022, are presented.

The CTTS AS 205 A = AS 205 N (V866 Sco) is a star of the spectral type K5 in a moderately bright ( ) triple system. At an angular distance of from AS 205 N (corresponding to 180 a.u. at the distance of 140 pc), there is a low-mass K7-M0 spectroscopic binary AS 205 S (Ghez et al., 1993; Prato et al., 2003). Two stable periods ( and ) are observed in the AS 205N light curve (Artemenko et al., 2010). The period is a typical one for CTTS and can be explained with the presence of cool spots on the stellar surface. The phase diagram of the period shows a modulation of brightness and color, indicating the presence of an additional cool source in the system. Since AS 205 N is about 2 times brighter than AS 205 S in the band (Herbig & Bell, 1988), the observed brightness modulation should be attributed to the main star. From the temperature and bolometric luminosity relations (Andrews et al., 2009), the mass of AS 205 N is expected to be about .

V1082 Cyg (HBC 728) is considered a typical CTTS, with a distance to the star of 660 pc. The brightness of the star is 133. Due to its being relatively faint, this star remained poorly studied. In the catalog by Herbig & Bell (1988), its number is HBC 728. Grankin et al. (2007) found for the star an average variability amplitude of 0828. Percy et al. (2010), mentioning the stars's photographic magnitude range 138 - 150, determined the range of its periodic variations , the period being 22. There is no accurate measurement of the star's spectral type because of its strong emission lines. Its average effective temperature is estimated as 5500 K.

The -band brightness of the Herbig AeBe star AS 310 (HBC 284) is 1252, and its variability is relatively poorly studied. No periodical brightness variations were detected (Percy et al., 2010). The spectral type of the star was determined as BIVe, with effective temperature at 25800 K (Hernández et al., 2004).

2. Observations

CCD photometry of the program stars was performed at the Cassegrain focus of the 60 cm Carl Zeiss telescope of the Shamakhy Astrophysical Observatory. As a light detector, an FLI  k CCD camera was used. The photometer is equipped with standard filters. After the previous modification of the photometer (Abdullayev et al., 2012, and references therein), several of parameters of the system were improved. Since our observations were obtained using the upgraded system, this paper describes the new photometer+telescope system which is currently in use.

The telescope, with an aperture of 600 mm and equivalent focal length  mm, has the scale in its focal plane per mm. Considering that the pixel size is 9m and the number of pixels per side is 4096, the size of the useful part of the focal plane is approximately .

With the given pixel size, we get 0 247 for single-pixel resolution. Depending on the seeing, or binning was used, corresponding respectively to 0 49 per pixel and 0 99 per pixel. The total area covered by the camera is approximately , and the effective linear area in the focal plane is . Dark, bias, flat-field calibration images were regularly taken during observations.

To transfer the instrumental system to the international Johnson-Cousins system, transition coefficients were determined based on observations of Landolt standards (Landolt, 1992). Currently, standard star fields are listed in the European Southern Observatory (ESO) (https://www.eso.org/sci/observing/tools/standards/Landolt.html) directory. The fields with Landolt standards were selected so that the brightness of the standards located there was close to the brightness of the program stars.

Diagrams in Fig. 1 show comparison between the brightness of standard stars and the instrumental brightness measurements obtained in different bands. For each panel of Fig. 1, the corresponding transition formulas and the reliability coefficient of the approximating straight line are given. As can be seen, the transition coefficients allow us to reduce the results to the international system with a high accuracy.

The formulas for converting our instrumental system to the international system are the following:

Here, the lowercase letters (right side) correspond to magnitudes in the instrumental system. When approximating the data, the reliability coefficient of passing a straight line through the data points was, on average, .

All reductions of the observations were performed using the MaxIm DL software. Typical average measurement errors for individual bands are , , , and .

Obtained observations

Our photometric measurements performed using the differential method. In Figs. 2-4, finding charts of the program stars are presented. R.m.s. errors of program-star photometry were determined with respect to standard and check stars. The magnitudes of the star AS 205 were determined relative to the reference star UCAC4 357-076076 whose magnitudes in each of the filters are the following: , , , . In order to check the stability of this star and the check stars C1 and C2, their magnitudes were derived relative to each other in different combinations in the available frames, and the stars showing stability in the derived errors were selected. The selected reference star (S) UCAC4 357-076076 and the check stars C1 and C2 are shown in Fig. 2. We used the same procedure for observations of the other program stars (Figs. 3-4). The mean rms of a measurement was calculated from photometric measurements of the check stars. In different filters, the following errors were found: , , , .

The results of our photometrical observations of the program stars are collected in Tables 1-3.

3. Conclusion

Of interest is the character of variations of the HBe star AS 310, observed in 2016-2022. Figure 5 presents its light curves in the and bands obtained in our observations. The maximal amplitudes registered in this time interval are , . Remarkably, the largest amplitude was observed for AS 310 in 2018 in the band. Unfortunately, at that time we did not observed the star in the band as frequently as in the band. Our observations show that, unlike other HAeBe stars, AS 310 has an unusually large amplitude of brightness variations, possibly due to the binarity or multiplicity of the star. The star's activity was different in different years of our observations. To clarify these findings, additional long-term observations of the star are needed.

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