Peremennye Zvezdy

Peremennye Zvezdy (Variable Stars) 44, No. 2, 2024

Received 15 March; accepted 12 April.

Article in PDF

DOI: 10.24412/2221-0474-2024-44-6-27

Seven New Multiperiodic δ Scuti stars in Cygnus

A. Samokhvalov

Surgut, Russia, e-mail: sav@surgut.ru


In a small field in Cygnus, I discovered seven new DSCTC stars that demonstrate multiperiodic pulsations from V-band CCD observations. For each of these stars, from two to four reliable pulsation frequencies could be derived. The paper presents finding charts, frequency spectra, light curves for program stars. New high-quality multi-band observations of the discovered variable stars with larger telescopes are needed.

1. Introduction

Working on our program aimed at discoveries and studies of DSCT-type stars, we have discovered seven new DSCTC stars with signs of multiperiodic pulsations in a field in Cygnus. The main information about these stars is presented in Table 1. Their coordinates were drawn from the Gaia DR3 catalog (Gaia Collaboration, 2022). None of these stars are currently contained in the General Catalogue of Variable Stars (GCVS), see Samus et al. (2017), or in the AAVSO Variable Star Index (VSX), see Watson et al. (2006). However, they are marked VARIABLE in the Gaia DR3 catalog, see Gaia Data Release 3 (Gaia DR3), Variability (Eier et al., 2023). Infrared color indices were drawn from 2MASS catalogue (Cutri et al., 2003).

Table 1. New Variable Stars
No. USNO-A2.0 RA, J2000.0 Dec, J2000.0 V J – H H – K J – K
1 1425-12331125
2 1425-12332333
3 1425-12344159
4 1425-12354548
5 1425-12372223
6 1425-12374410
7 1425-12377701

Based on VSX and 2MASS catalogs, we plot a population distribution diagram of DSCT stars (DSCT, DSCTC, and HADS-type stars) as a function of infrared colors, derived from 2MASS photometry (Fig. 1). Only stars with good 2MASS photometry () and reliably defined variability types (without an uncertainty symbol ":" in VSX Type) are used. All the new variable stars reported here also possess the 2MASS quality flag . The new variable stars are located in this diagram near the maximum-density population core, in the green and blue zones. This position can be considered one of the signs of belonging to the DSCT variability type.

Fig. 1. Population distribution diagram of DSCT stars as a function of 2MASS infrared colors and . New variable stars are marked in the diagram.

2. Observations, primary reduction and magnitude calibration

Our observations were carried out at the Caucasian Mountain Observatory (CMO) of M.V. Lomonosov Moscow State University (Shatsky et al., 2020) using the 0.25-m remote-controlled Ritchey-Chretien telescope, equipped with a SBIG STXL-6303e CCD Camera and a filter. A total of 1120 images of the field with 600-second exposures were obtained on JD 2460187-2460325. Information about the number of images taken on each observing night is given in Table 2.

Table 2. Images taken on each observing night
HJD Images HJD Images HJD Images HJD Images HJD Images
2460187 12 2460211 27 2460237 33 2460259 33 2460302 4
2460188 9 2460212 4 2460238 30 2460261 13 2460308 23
2460189 15 2460216 25 2460240 29 2460275 5 2460309 6
2460190 14 2460218 32 2460246 21 2460277 31 2460310 9
2460198 22 2460223 9 2460248 35 2460281 37 2460311 21
2460202 18 2460225 33 2460249 32 2460282 15 2460314 14
2460203 16 2460226 6 2460250 19 2460283 36 2460316 18
2460207 23 2460230 30 2460253 31 2460286 5 2460320 22
2460208 25 2460231 30 2460254 15 2460287 5 2460321 21
2460209 26 2460232 30 2460256 12 2460298 28 2460325 17
2460210 28 2460233 30 2460257 11 2460299 25

For basic reductions for dark current, flat fields, and bias, and in order to remove hot pixels and cosmic rays hits, we used IRAF routines and proprietary software TheSkyXTMby Software Bisque Inc. For calibration, each observing night we obtained 16 bias frames, 16 dark frames, 16 flat fields, plus 16 dark frames corresponding to flat fields.

For photometry of new pulsating stars, we applied VaST software by Sokolovsky and Lebedev (2018). All times in this paper are expressed in terrestrial time in accordance with IAU recommendations (resolution B1 XXIII IAU GA), with heliocentric corrections applied.

For magnitude calibration in the band, we use data of the GAIA DR3 catalogue. We restrict ourselves to single, relatively bright stars, with no saturation of pixels for our CCD camera, no close neighbors, and demonstrating no brightness variations during the time interval of our observations. Detailed information about our calibration stars is collected in Table 3. Uncertainties in the column were derived from our photometry, the GAIA , , and magnitudes were drawn from the corresponding catalog. Magnitudes in the "Calc. " column were obtained using the equation:

 
(1)

which is based on table 5.9 of the Gaia Data Release 3, Documentation release 1.2
(https://gea.esac.esa.int/archive/documentation/GDR3/)

Table 3. Magnitudes of calibration stars
      GSC       σV GAIA  Calc. V 
G GBP GRP
3968-2621 0.008 11.5102 12.5060 10.5323 12.2394
3968-3272 0.007 12.1790 12.5445 11.6340 12.3610
3968-3000 0.008 12.3394 12.5281 12.0156 12.4139
3968-2894 0.007 12.0446 12.3314 11.5923 12.1732
3968-2595 0.007 11.8378 12.1974 11.3031 12.0143
3968-2517 0.007 12.0227 12.2059 11.7003 12.0958

The observations are presented as a zip archive in the html version of this paper.

3. Results

To derive periods, we use Period04 software by Lenz and Breger (2005) that implements the discrete Fourier transform, very suitable for analysis of sine-shaped light curves of the pulsating variable stars with multiperiodicity.

We searched for periodic signals in our observations in the frequency range between 3 and 20 cycles per day that was selected following recommendations by Breger (2000). We continuously calculate significant frequencies, in the first iteration based on the original data, in the following iterations, using residuals instead, as long as the signal-to-noise ratio for the corresponding peak in the Fourier frequency spectrum exceeds 4. This is the empirical criterium obtained from the analysis of observations by Breger et al. (1993) and ensuring that the signal is a real feature. Parameters of the oscillations corresponding to the equation

(2)

were determined by least squares and are collected in Table 4. The first column of this table gives the star's number in the USNO-A2.0 catalogue (Monet et al., 1998). The second column is the number of photometric measurements for the corresponding star. The third column is the average error of photometrical measurements. The fourth column contains the mean magnitude corresponding to Equation 2. Subsequent columns describe oscillations of each star. The column named contains numbers of significant frequencies, which are presented in the next column. Columns marked and contain the phase and amplitude of the th oscillation. In the last column, we give the ratio for the th frequency, derived using the Period04 software. Only frequencies that satisfy the criterion are kept.

For plotting light curves, Fourier spectra, and population distribution diagram, we used our own routines written in Python 3 programming language using the NumPy (Harris et al., 2020) and Matplotlib (Hunter, 2007) libraries.

Table 4. Detected frequencies of the new pulsating variable stars
   USNO A2.0       N    merror mmean Oscillations
Freqi  Frequency, d-1  Φi  Ai, mag   SNR 
1425-12331125 986 0.012 15.4382 f1 7.17082 0.11158 0.0213 23.48
f2 8.46182 0.33195 0.0042 4.80
1425-12332333 1051 0.0018 12.3762 f1 5.841265 0.28593 0.0123 26.98
f2 5.503670 0.91417 0.0060 12.73
f3 7.543690 0.55110 0.0039 10.27
f4 6.930815 0.05636 0.0031 7.91
1425-12344159 1054 0.013 15.5964 f1 11.349167 0.16114 0.0173 15.00
f2 6.429910 0.95664 0.0163 12.13
f3 11.116893 0.30195 0.0104 8.99
f4 11.034570 0.09757 0.0066 5.63
1425-12354548 1057 0.0033 13.5234 f1 17.932155 0.85287 0.0107 30.26
f2 18.073310 0.02408 0.0037 10.46
f3 13.329770 0.57958 0.0019 5.45
f4 14.873727 0.49255 0.0018 5.13
1425-12372223 1059 0.0156 15.8190 f1 8.979444 0.15165 0.0285 23.99
f2 8.713564 0.07668 0.0270 22.54
f3 9.410159 0.43088 0.0196 16.52
f4 17.201639 0.96931 0.0085 7.70
1425-12374410 1058 0.0054 14.3492 f1 12.666880 0.58088 0.0159 21.03
f2 12.963420 0.33383 0.0084 10.76
1425-12377701 1054 0.0097 15.2028 f1 17.573930 0.13065 0.0159 24.92
f2 14.945790 0.93866 0.0080 10.64
f3 14.717230 0.02628 0.0045 5.92

Our observations are available as a zip archive in the html version of this paper.

3.1. USNO-A2.0 1425-12331125

This is the faintest star in our sample, and we found only two significant frequencies for it. The second frequency has a very small amplitude and is very close to the lower reliability boundary, with the ratio 4.8.

Figure 2 presents the frequency spectrum of USNO-A2.0 1425-12331125 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations.

Fig. 2. The frequency spectrum and light curve of USNO-A2.0 1425-12331125. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 3.

Fig. 3. A finding chart for USNO-A2.0 1425-12331125.

The -band phased light curve of USNO-A2.0 1425-12331125 with the following light elements:


is presented in Fig. 4.

Fig. 4. Phased light curve of USNO-A2.0 1425-12331125.

3.2. USNO-A2.0 1425-12332333

Figure 5 presents the frequency spectrum of USNO-A2.0 1425-12332333 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations. Light curve variations are easy to notice, they are reproduced with the model rather well.

Fig. 5. The frequency spectrum and light curve of USNO-A2.0 1425-12332333. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 6.

Fig. 6. A finding chart for USNO-A2.0 1425-12332333.

The phased -band light curve of USNO-A2.0 1425-12332333 with the following light elements:


is presented in Fig. 7.

Fig. 7. Phased light curve of USNO-A2.0 1425-12332333.

3.3. USNO-A2.0 1425-12344159

The frequency spectrum of USNO-A2.0 1425-12344159 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations are given in Fig. 8. Light curve variations are easy to notice, they are reproduced with the model rather well.

Fig. 8. The frequency spectrum and light curve of USNO-A2.0 1425-12344159. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 9.

Fig. 9. A finding chart for USNO-A2.0 1425-12344159.

The phased -band light curve of USNO-A2.0 1425-12344159 with the following light elements:


is presented in Fig. 10.

Fig. 10. Phased light curve of USNO-A2.0 1425-12344159.

3.4. USNO-A2.0 1425-12354548

Figure 11 presents the frequency spectrum of USNO-A2.0 1425-12354548 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations.

Fig. 11. The frequency spectrum and light curve of USNO-A2.0 1425-12354548. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 12.

Fig. 12. A finding chart for USNO-A2.0 1425-12354548.

The phased -band light curve of USNO-A2.0 1425-12354548 with the following light elements:


is presented in Fig. 13.

Fig. 13. Phased light curve of USNO-A2.0 1425-12354548.

3.5. USNO-A2.0 1425-12372223

Figure 14 presents the frequency spectrum of USNO-A2.0 1425-12372223 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations.

Fig. 14. Frequency spectrum and light curve of USNO-A2.0 1425-12372223. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 15.

Fig. 15. A finding chart for USNO-A2.0 1425-12372223.

The -band phased light curve of USNO-A2.0 1425-12372223 with the following light elements:


is presented in Fig. 16.

Fig. 16. Phased light curve of USNO-A2.0 1425-12372223.

3.6. USNO-A2.0 1425-12374410

Figure 17 presents the frequency spectrum of USNO-A2.0 1425-12374410 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations.

Fig. 17. The frequency spectrum and light curve of USNO-A2.0 1425-12374410. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 18.

Fig. 18. A finding chart for USNO-A2.0 1425-12374410.

The phased -band light curve of USNO-A2.0 1425-12374410 with the following light elements:


is presented in Fig. 19.

Fig. 19. Phased light curve of USNO-A2.0 1425-12374410.

3.7. USNO-A2.0 1425-12377701

Figure 20 presents the frequency spectrum of USNO-A2.0 1425-12377701 and its theoretical light curve (solid curve) with superposed data points corresponding to individual observations.

Fig. 20. The frequency spectrum and light curve of USNO-A2.0 1425-12377701. In the bottom panel, the solid curve is the synthesized light curve and dots are observed data points.

The finding chart based on a POSS2 red plate is presented in Fig. 21.

Fig. 21. A finding chart for USNO-A2.0 1425-12377701.

The phased -band light curve of USNO-A2.0 1425-12377701 with the following light elements:


is presented in Figure 22.

Fig. 22. Phased light curve of USNO-A2.0 1425-12377701.

4. Conclusion

We have found seven new pulsating DSCTC stars with reliable signs of multiperiodic pulsations. All detected frequencies correspond to reliable oscillations.

The fact that the studied seven DSCTC stars are located in a small field (0.42 square degrees) is a good reason to continue the search for stars with similar pulsation properties at low galactic latitudes, in adjacent star fields.

In order to check for the presence of possible additional frequencies and to verify pulsation mode identification, new precision photometry in several standard photometric bands is needed. We appeal to observers having access to large telescopes to continue observations of these interesting variable stars.

Acknowledgements: I would like to thank Prof. N. N. Samus for helpful discussion and the anonymous referee for suggested improvements of the manuscript.

References:

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