Peremennye Zvezdy (Variable Stars) 42, No. 11, 2022 Received 5 November; accepted 23 November.	Article in PDF
	DOI: 10.24412/2221-0474-2022-42-87-105

Long-term Brightness and Color Variations of the Young Variable Star V645 Cyg

S. Yu. Gorda, A. P. Bisyarina, A. M. Sobolev, S. Yu. Parfenov

Kourovka Astronomical Observatory, Ural Federal University, Mira str. 19, Yekaterinburg, 620002 Russia

Variability of V645 Cyg was discovered by Hoffmeister et al. (1951). Cohen (1977) found that the position of V645 Cyg coincided, within , with a reflection nebula and found three condensations (N0, N1 and N2 in his notation) from spectrophotometry and spectropolarimetry. He came to the conclusion that the star itself, whose radiation is reflected by the condensations, is a young star of the spectral type O and, most likely, is situated between the starlike knot N0 and the knot N1, partially being in a gas-dust cocoon. Further studies of the object V645 Cyg, mainly in the infrared (IR) and radio ranges (Humphreys et al., 1980; Lada et al., 1981; Morris et al., 1982; Torrelles et al., 1983; Goodrich, 1986), revealed bipolar gas outflows, suggestive of the presence of a molecular protoplanetary disk observed in the IR range and in the maser lines of CH₃OH and H₂O (Valtts et al., 2002). The maser emission in OH transitions was also detected (Morris et al., 1982).

More recent mid-infrared studies of V645 Cyg have shown that N0 is a point source, i.e. a young star, and the nebular component N1, located five arc seconds northwest of N0, is a diffuse nebula, especially since the N0 component is 200 times brighter than N1 (Clarke et al., 2006). Thus, it became clear that N0 was the star V645 Cyg itself, and not just a knot of a reflection nebula such as N1, as it had been previously thought. Various observations show that V645 Cyg is a very complex and interesting object to study, containing a young massive star.

Despite the general opinion that V645 Cyg is a young stellar object, there is still no consensus on its physical parameters. Some authors prefer the classification of an O-type star (e.g., Testi et al., 1998), while others consider it an A-type supergiant (e.g., Bowey et al., 2003). The distance to the object was also ambiguous, ranging from 3 to 6 kiloparsecs (Clarke et al., 2006). The first measurement of trigonometric parallax from H₂O maser observations corresponds to a distance of 5.28 kpc (Honma et al., 2007), but later measurements indicate a smaller value of 4.054-4.15 kpc (Asaki et al., 2014; Quiroga-Nuñez et al., 2019).

According to one of the recent studies (Miroshnichenko et al., 2009), V645 Cyg is a young massive B-type main sequence star that has already passed the stage of protostellar accretion and is being released from its gas-dust cocoon. Such young stellar objects are rare because they are quite massive and evolve rapidly. They are usually deeply embedded in the dust shell and their radiation manifests itself only in the IR range (Zinnecker et al., 2007). V645 Cyg is one of the few objects of this class visible in optics. One of the reasons for this is the orientation of the circumstellar disk plane, which practically coincides with the sky plane (Valtts et al., 2002; Miroshnichenko et al., 2009). Optical and IR images of the star contain two bright regions, which are interpreted as radiation from the disk and envelope (Cohen, 1977; Goodrich, 1986).

The angular size of the IR image increases with wavelength in the near- and mid-IR and decreases towards the millimeter range (Quiroga-Nuñez et al., 2019; Miroshnichenko et al., 1999). Based on medium-resolution spectroscopy data, Hamann and Persson (1989) suggested that these bright regions could represent reflected light from the disk and bipolar outflows.

The study of the V645 Cyg brightness changes was carried out mainly in the IR range. There are only two series of observations in the optical range, these are the very first photographic observations of Hoffmeister et al. (1951) in the 1950s and photoelectric observations by Shevchenko et al. (1993) in 1985-2000. Miroshnichenko et al. (2019), based on the analysis of the photometric data from Shevchenko et al. (1993) obtained in the filters at the Maidanak observatory, indicate the possible presence of an 8-year variability period of the star's optical brightness with an average amplitude in the band and a decrease of the IR brightness at time scales of the order of 20 years. Interpreting the speckle interferometry data, Miroshnichenko et al. (2009) subdivided the -band image of the star into two components. The bright, small ring component (radius a.u.) is probably the emission from the disk near the sublimation front. The second component ( a.u.) may correspond to the emission of the circumstellar shell's inner part, but the authors emphasize that one cannot exclude, for example, this emission being light reflected off the boundary of the cavity formed by bipolar outflows.

Since photometric observations of V645 Cyg in the optical range were very rare, and the latest of them date back to 2000, new observations are extremely useful, at least in terms of confirming the periodic brightness variation discovered by Miroshnichenko et al. (2009).

Our optical photometric observations of V645 Cyg were carried out on 111 nights from May, 2009 to March, 2018 using the AZT-3 reflector ( m,  m) and from December, 2018 to April, 2022, using the AstroSib-500RC Ritchey-Chretién telescope ( m,  m) of the Kourovka Astronomical Observatory (Ural Federal University). Observations were carried out in the filters, reproducing a system close to the Johnson-Cousins system when observing with Apogee Alta U6 CCD cameras and a Kodak KAF-1001E chip ( , 24 m) and, since 2015, with a PL A230 camera of the company FLI and an E2V 230-42 CCD chip ( , 15 m). Two stars were chosen as a comparison star and a check star in the immediate vicinity of the variable, the data on them are given in Table 1. The coordinates in the Table are based on the Gaia-DR3 catalog.

After photometric processing, which included subtracting dark frames, dividing into flat-field frames, and determining the brightness of stars by aperture photometry, we obtained 111 night-averaged values of the brightness differences between V645 Cyg and the comparison star in the filter, 96 values in the filter, and 58 values in the filter. The aperture size was chosen to cover the whole radiation associated to V645 Cyg in the given filter (including components N0 and N1). The light curves for V645 Cyg obtained over the 13-year observation period are shown in Fig. 1, Fig. 2 and Fig. 3. Data are given in Tables 2-6.

As can be seen, a systematic star brightness increase was observed throughout 13 years of our observations along with brightness changes of V645 Cyg with amplitudes from several hundredths to tenths of a magnitude over time intervals from a day to several months. At the same time, the brightness of the comparison star, as can be seen in Fig. 1, Fig. 2 and Fig. 3, remained practically unchanged within the limits of observational errors. Also, the average brightness of V645 Cyg in the band has increased over 13 years by , in the band by , and in the band by .

It can be noted that, along with an increase in brightness, the reddening of the star is also observed, which is clearly seen in the plots of and color indices change over time, shown in Fig. 4 and Fig. 5, respectively. Over the entire observation period, the color index increased by , and by . A change in the optical brightness within was also observed earlier in the data obtained by Shevchenko et al. (1993) (see Miroshnichenko et al., 2009). However, the same paper notes that the color indices , as well as and , remained stable within , which is significantly less than the amplitude of the changes in the data obtained from observations at the Kourovka Observatory.

We also did not find any periodicity in the brightness variations of V645 Cyg, especially with a period of 8 years, which was found by Miroshnichenko et al. (2009) in the observations of Shevchenko et al. (1993). Apparently, the periodic change in the optical brightness of this star occurs only during a certain time interval and is not a constant phenomenon.

It is important to note that optical brightness variations in the time interval are closely followed by the variations of CHOH maser flux at 6.7 GHz presented by Szymczak et al. (2018). The increase in optical brightness corresponds to the increase of maser brightness and vice versa. Szymczak et al. (2018) found that the maser flux variations were quasi-periodic with a period of 195 days. Based on the observed correspondence between the maser and optical brightness variability, it can be assumed that the optical brightness variations at time scales of the order of 100 days are also quasi-periodic. As shown by Kojima et al. (2018) for the object G33.641-0.228, CHOH maser bursts can be observed on time scales of the order of several hours. Taking into account that the propagation speed of radiation, which affects the pumping of masers, can be significantly lower than the speed of light (Burns et al., 2020) and that the relaxation time of the CHOH level populations can be of hours, the interpretation of such maser variability that fast may require to take into account the non-stationary radiation transfer and excitation in the CHOH transitions. Our data shows that changes in the optical brightness during a night are mostly monotonic (see also Sobolev et al., 2013). Thus, based on our data in the optical range, we can assume that there are no strong sudden changes of CHOH masers in V645 Cyg on time scales of the order of several hours.

The data obtained by us during thirteen years of photometric observations of the young irregular variable star V645 Cyg are, in essence, the second monitoring series obtained in the optical range in the process of studying this star using modern detectors. In this period of observations, in contrast to the results of the earlier optical monitoring of V645 Cyg (see Shevchenko et al., 1993), along with changes in brightness, a significant change in the color indices towards increasing reddening of the star was also observed.

Based on the results of this series of observations, as well as on data obtained at the same time in the IR range (Shenavrin et al., 2011), a number of conclusions can be drawn about the reasons for such changes in the brightness and color of V645 Cyg. These changes can be due to a change in the angle of inclination of the disk plane due to precessional rotation, during which hot structures in the inner region of the disk become more visible to the observer. Another possible scenario explaining the variability in optics and IR can be similar to that presented by Parfenov & Sobolev (2014). In this scenario, a spiral density wave in the inner disk region formed due to multiplicity of the system can propagate up to the radius of the sublimation front. There is a layer of hot gas at the end of the spiral wave near the surface of the star. During rotation, this heated layer is on the line of sight, the hot substance can approach the gas and dust inhomogeneities on the inner boundary of the disk and heat them up. This scenario, however, assumes that the variability of the CHOH maser flux at 6.7 GHz can be different for different components of the maser spectrum, which are formed in different regions in the vicinity of the star. The observed variability of the maser emission, which is in good agreement with optical brightness variations at time scales of the order of 100 days, is the same in all components of the maser spectrum. Another scenario that can be used to explain the observed variability both in the optical range and in maser lines suggests variations in the accretion rate of matter onto the star, as a result of which the luminosity and temperature of the star change. These changes, in turn, lead to a change of temperature of the dust, radiation of which pumps the CHOH masers (Sobolev & Deguchi, 1994). This scenario contradicts the conclusion about the absence of accretion made by Miroshnichenko et al. (2009) based on spectroscopic observations of V645 Cyg. However, it can be that the spectroscopic observations of Miroshnichenko et al. (2009) were carried out at the time of a significant drop in the accretion rate. And finally, the fourth reason for the manifestation of the observed changes in the brightness and color of V645 Cyg in the observed time interval may be further dissipation of the gas-dust cocoon surrounding the star and motion of the star out of cocoon which makes radiation from the star and its heated immediate vicinity more visible. This scenario does not allow one to explain optical brightness variations at time scales of the order of 100 days, which can be quasi-periodic, as well as the corresponding variations in the maser flux density. However, this scenario is applicable to explain the observed monotonic variations in brightness and color of V645 Cyg.

Acknowledgments. A.P.B, A.M.S. and S.Yu.P. were supported by the Foundation for the Advancement of Theoretical Physics and Mathematics "BASIS". S.Yu.G was supported by the Ministry of Science and Higher Education of the Russian Federation under contract 075-15-2020-780.

References:

Asaki, Y., Imai, H., Sobolev, A. M., Parfenov, S. Yu., 2014, Astrophys.J., 787, article id. 54, 9 pp.

Bowey , J. E., Adamson A. J., & Yates, J. A., 2003, Mon. Notices Roy. Astron. Soc., 340, 1173

Burns, R. A., Sugiyama, K., Hirota, T., et al.,2020, Nature Astronomy, 4, 506

Clarke, A. J., Lumsden, S. L., Oudmaijer, R. D., et al., 2006,Astron. & Astrophys., 457, 183

Cohen, M., 1977, Astrophys. J., 215, 533

Goodrich, R. W., 1986, Astrophys. J., 311, 882

Kojima, Y., Fujisawa, K., & Motogi, K., 2018, Astrophysical Masers: Unlocking the Mysteries of the Universe, Proceedings of the International Astronomical Union, IAU Symposium, 336, 336

Hamann, F. & Persson, S. E., 1989, Astrophys. J., 339, 1078.

Hoffmeister, C., Rohlfs, E., & Ahnert, P., 1951, Veröff. Sternwarte Sonneberg, 1, 407

Honma, M., Bushimata, T., Choi, Y. K., et al., 2007, Publ. Astron. Soc. Japan., 59, 889

Humphreys, R. M., Merrill, K. M., & Black, J. H., 1980, Astrophys. J., 237, L17

Lada, C. J., Blitz, L., Reid, M. J., & Moran, J. M., 1981, Astrophys. J., 243, 769

Miroshnichenko, A. S., Hofmann, K.-H., Schertl, D., et al., 2009, Astron. & Astrophys., 498, 115

Miroshnichenko, A., Ivezic, Z., Vinkovic, D., & Elitzur, M., 1999, Astrophys. J., 520, L115

Morris, M. & Kazes, I., 1982, Astron. & Astrophys., 111, 239

Parfenov, S. Yu. & Sobolev, A. M., 2014, Mon. Notices Roy. Astron. Soc., 444, 620

Quiroga-Nuñez, L. H., Immer, K., van Langevelde, H. J., et al., 2019, Astron. & Astrophys., 625, id. A70, 12pp.

Shenavrin, V. I., Taranova, O. G., & Nadzhip, A. E., 2011, Astronomy Reports., 55, 31

Shevchenko, V. S., Grankin, K. N., Ibragimov, M. A., et al., 1993, Astrophys. Space Sci., 202, 121

Sobolev, A. M. & Deguchi, S., 1994, Astron. & Astrophys., 291, 569

Sobolev, A. M., Gorda, S. Yu., & Davydova, O. A., 2013, Inform. Bull. Var. Stars, No. 6061

Szymczak, M., Olech, M., Sarniak, R., et al., 2018, Mon. Notices Roy. Astron. Soc., 474, 219

Testi, L., Palla, F., & Natta, A. A,, 1998, Astron. & Astrophys. Suppl. Ser., 133, 81

Torrelles, J. M., Rodriguez, L. F., Canto, J., et al., 1983, Astrophys. J., 274, 214

Val'tts, I. E., Slysh, V. I., Voronkov, M. A., & Migenes, V., 2002, Astron. Soc. Pacific Conference Series, 279, 279

Zinnecker, H. & Yorke, H. W., 2007, Ann. Rev. Astron. Astrophys., 45, 481

---