Peremennye Zvezdy (Variable Stars) 45, No. 17, 2025 Received 1 December; accepted 11 December.	Article in PDF
	DOI: 10.24412/2221-0474-2025-45-177-184

USNO-B1.0 1491-0020709 - A New Double-mode High-Amplitude  Scuti Star

Sergei V. Antipin¹, Alexandra M. Zubareva^2,1, Natalia P. Ikonnikova¹, Alexandre A. Belinski¹, Marina A. Burlak¹, Nikolay N. Samus^2,1

1. Sternberg Astronomical Institute, Lomonosov Moscow University, Universitetskij pr. 13, 119234 Moscow, Russia

2. Institute of Astronomy of the Russian Academy of Sciences, Pyatnitskaya str. 48, 119017 Moscow, Russia

1. Introduction

The classical Scuti variables represent a group of pulsating stars located in the lower part of the Cepheid instability strip in the Hertzsprung-Russell diagram, at spectral types A0-F5. Their brightness changes in the range from 0003 to 09 in band, with periods from 0 01 to 0 2 (Samus et al. 2017). At the same time, Breger (2000) reported different period intervals, from 002 to 025. Stars with higher amplitudes (more than 025) are assigned to the subgroup of high amplitude  Sct stars (HADS). Many of them pulsate at least in two modes: the fundamental mode () and the first overtone (). The period ratios are found to be in range 0.76 to 0.78 and depend on masses and metallicities of the stars (Petersen and Christensen-Dalsgaard 1996; Yang et al. 2021). The  Sct-type stars of solar metallicities have masses from (short-period stars) to (long-period stars) (McNamara 2011).

The variability of USNO-B1.0 1491-0020709 (RA = 003919103, Dec = +590637 81 J2000.0; Gaia Collaboration 2023) was discovered by us in 2013 in the course of the program of scanning photographic plates of Moscow stacks¹ and searching for variable stars in the images (e.g. Antipin et al. 2018).

We found out that the object belonged to double-mode HADS stars already from photographic photometry, but the accuracy of the photographic photometry was not sufficient to publish its light curve. That was the reason to start more precise CCD photometry.

Independently Chen et al. (2020) also discovered the object as a variable and designated it ZTF J003919.09+590637.9 while preparing the catalog of variable stars of the Zwicky Transient Facility (ZTF) project. The authors classified the star as high-amplitude  Sct object with uncertainty (HADS:) or an eclipsing system of W UMa type. Their catalog suggests only one period, 01857867.

2. Observations and data reduction

Our photometric observations were carried out in 2014-2023. We acquired 2897 individual measurements on 107 nights. In 2014 and 2023 one of the authors (A. Z.) collected 1050 frames with the Zeiss-600 telescope of the Crimean Astronomical Station of the Sternberg Astronomical Institute of M. V. Lomonosov Moscow State University. An Apogee AP-47p CCD detector was used in 2014 with exposure times of 180 seconds, and FLI16803 with exposure times of 120 seconds was used in 2023. In 2020 and 2021, we used the RC600 telescope of the Caucasus Mountain Observatory of M. V. Lomonosov Moscow State University (Berdnikov et al. 2020) equipped with Andor iKon-L (DZ936N-BV) CCD. The images were also taken in the band, exposure times were 120 seconds. A. B., M. B., and N. I. acquired 1847 frames. Initial data reductions included standard operations: the images were corrected by subtracting biases and dark current, and then divided by flat field frames. The aperture photometry and the magnitude calibration were performed by means of VaST² software package (Sokolovsky & Lebedev 2018). Magnitudes of the stars in the field of view were drawn from the APASS catalog³. The mean brightness of USNO-B1.0 1491-0020709 is . The typical errors are close to 0004. The overall light curve is presented in Fig. 1, and the log of observations is summarized in Table 1.

3. Periodicities

We used Deeming (1975) method implemented in the WINEFK⁴ package by Dr. V. P. Goranskij to search for periodicities. The resulting periodogram, where two significant peaks are marked, is given in Fig. 2. The phased light curves for these two periods are presented in Fig. 3a and Fig. 3c. The light elements for the fundamental mode and first overtone are as follows:

We analyzed the power spectra of USNO-B1.0 1491-0020709 to derive possible interaction frequencies and subtract them from our data. Table 2 contains the information about the characteristic values of the signals. The corresponding prewhitened light curves for the first overtone and fundamental mode are presented in Figs. 3b and 3d.

We found that USNO-B1.0 1491-0020709 exhibited the lowest period ratio for double-mode HADS stars: = 0.7598, which is slightly below the typical interval for , which is 0.76-0.78 (Petersen & Christensen-Dalsgaard 1996; Yang et al. 2021). The statistics of double-mode HADS stars (e.g. their distribution by periods) is quite incomplete. As of the end of 2025, it is very difficult to take into account all relevant publications with sufficient numbers of double-mode HADS stars. From several publicly available surveys, especially the TESS mission, more than 2400 double-mode (or even multimode) HADS stars are known. In some cases, authors do not subdivide their HADS stars into single-mode, two-mode, or three-mode pulsator groups, making it difficult to quickly locate needed objects in their lists. Yang et al. (2021) released a catalog containing 144 double-mode HADS stars pulsating in the fundamental mode and the first overtone. The catalog is a compilation of previously published pulsating stars from papers of different researchers. Khruslov (2021, 2022, 2023, 2024) released four catalogs of double- and multi-mode pulsating variables containing almost 250 HADS stars. Less than twenty HADS stars in these catalogs have fundamental-mode periods exceeding 0.2 days. Jia et al. (2024) reported more than 2100 newly discovered double-mode HADS stars from the ZTF data archive. Xue et al. (2023) published a study of 27 double-mode HADS variables found in the TESS mission data.

4. Mass determination using the Petersen's diagram

Masses of double-mode pulsating variable stars can be estimated with no information about their luminosities and effective temperatures. The two independent periods derived from observations are sufficient for this purpose. The ratio of the star's period in the first overtone and that in the fundamental mode (or, simpler, of the shorter period to the longer one) should be in a narrow interval that depends on the mass of the variable. Petersen (1973) was the first to apply this idea to double-mode Cepheids by comparing theoretical computations to observational data. Later Petersen & Christensen-Dalsgaard (1996) demonstrated applicability of the approach to high-amplitude  Scuti stars. The typical values for are 0.76-0.78.

Based on Petersen's diagram for HADS stars from TESS published by Xue et al. (2023), we can give a quite precise estimate for the mass of USNO-B1.0 1491-0020709. We reproduce the upper panel of Fig. 8 from Xue et al. (2023) and place there our object, marked with a star. Knowing the period ratio for USNO-B1.0 1491-0020709 from our CCD observations, we plot the object in the diagram in accordance with its periods corresponding to =0.7598 and . The resulting mass estimate is in range 1.9-2 (see Fig. 4). Note that the evolutionary tracks in Fig. 8 from Xue et al. (2023) were computed for a certain fixed metallicity (see the discussion therein). If we try to place USNO-B1.0 1491-0020709 with and =0.7598 in Petersen's diagram combined with evolutionary tracks published in Fig. 4 from Poretti et al. (2005), we get a mass estimate slightly exceeding 2  (Fig. 5). Poretti et al. (2005) used a different approach to constructing evolutionary tracks and considered effects of metallicity changes. Determinations of HADS masses with high accuracy by means of Petersen's diagrams from theory and observations are not straightforward. The estimates are good for quite massive double-mode (or multi-mode) puslators, as in the case of USNO-B1.0 1491-0020709. But for double-mode HADS stars with ( ), the theoretical Petersen's diagrams almost coincide with each other no matter the mass of the star given the same metallicity (for example, see Poretti et al. 2005 and Xue et al. 2023).

5. Conclusions

We performed observations and analysis of photometric data for a high-amplitude  Scuti type star USNO-B1.0 1491-0020709 in the time interval between 2014 and 2023. We analyzed our data for periodicities with the help of WINEFK package and found two periods, and . Using Petersen's diagram that relates periods, period ratios, and masses for double-mode pulsators including HADS variables, we estimated the mass of USNO-B1.0 1491-0020709 as . Among many hundreds of currently known double-mode HADS stars (Khruslov 2021, 2022, 2023, 2024; Yang et al. 2021; Xue et al. 2023; Jia et al. 2024), only several per cent of objects have fundamental periods close to 0 25, which corresponds to masses . High-accuracy mass estimation for double-mode HADS stars using the observational and theoretical Petersen's diagrams is quite possible for massive pulsators.

Acknowledgments. The authors are grateful to Dr. K. V. Sokolovsky for his VaST software package and enormous multi-skilled efforts which made the study of objects in scanned images from Moscow Plate Archive feasible.

The study was conducted under the state assignments of Lomonosov Moscow State University and of the Institute of Astronomy (Russian Academy of Sciences). The work was supported in part by M. V. Lomonosov Moscow State University Program of Development.

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