Research Article  Open Access
New Photometric Investigation of the SolarType ShallowContact Binary HH Bootis
Abstract
The shortperiod solartype contact binary HH Boo was monitored photometrically for about 8 years. It is found that the CCD light curves in the , , , and bands obtained in 2010 are symmetric, while the multicolor light curves observed in 2011 and 2012 by several investigators showed a positive O’Connell effect where the maxima following the primary minima are higher than the other ones. This indicates that the light curve of the solartype contact binary is variable. By analyzing our multicolor light curves with the WilsonDevinney code (WD code), it is confirmed that HH Boo is a Wtype shallowcontact binary system with a mass ratio of = 1.703(31) and a degree of contact factor of = . By including 109 new determined times of light minimum together with those compiled from the literature, it is detected that the diagram shows a cyclic oscillation with a period of = 6.58(11) yr and an amplitude of = 0.0018 d. The cyclic change may reveal the presence of an extremely cool third body orbiting the central binary.
1. Introduction
HH Boo (GSC0347200641, NSVS 5100852) was first listed as a star with a magnitude of = and a colour index of = in the TYCHO2 Catalogue [24]. The variability of HH Boo was discovered by Maciejewski et al. [2]. They reported that HH Boo was a EWtype binary system with a period of almost 8 hours from 214 data points collected during 12 nights between April 19 and May 7, 2003. The depths of the primary and secondary minima on their light curves are = and , respectively. They gave a preliminary ephemeris for the primary minimum asThe spectrum of HH Boo is most similar to G5III type from their optical spectra in the blue. The first radialvelocity studies of HH Boo have been done by Maciejewski & Ligeza [25]. They estimate the radii and the masses and derive a mass ratio = 0.633(42), = 0.78(8) , and = 0.49(5) . Recently, a new period distribution for EWtype binaries was given by Qian et al. [26, 27] based on the orbital periods of 40646 systems given in VSX (the international variable star index [28]). The period (0.319 d) of HH Boo is close to the peak of the distribution for EWs indicating that it is a typical EWtype binary.
The first photometric multicolor light curves in , , and bands were published by Dal & Sipahi [16] that were obtained with 35 cm SchmidtCCassegrain type MEADE telescope at the Ege University Observatory. As shown in Figure 3 in their paper, their light curves showed a positive O’Connell effect [29] where the maxima following the primary minima are higher than the others. The asymmetries of the light curves may be caused by stellar darkspot activities (e.g., [30]). They demonstrated that HH Boo is most likely a member of the Atype subclass of W UMa binaries and derived absolute parameters of HH Boo. A continuous decrease in the orbital period with a rate of = d was detected by them that was explained by either mass transfer from the secondary to the primary or mass loss from the system.
HH Boo was later observed in 2011 and 2012 by using the 0.40 m MeadeLX200 GPS telescope at Ankara University Observatory [31] and new CCD light curves in bands were obtained. Their light curves also show positive O’Connell effect. By analyzing their light curves and published radial velocity data, they determined the parameters of the binary. They found that HH Boo is Wsubclass contact system and the asymmetry of the light curves was interpreted by one cool star spot region located on the primary star (the hotter, less massive component). They reported that the diagram showed a cyclic variation with a period of = 7.39 yr and an amplitude of = 0.00227 d.
2. Observations
2.1. Spectroscopy
To determine the spectrum of the solartype contact binary, the lowresolution spectrograms for HH Boo were observed by using the OMR spectrograph of the 2.16 m telescope at Xinglong station of National Astronomical Observatories (Xinglong2.16m) in China on 2017 June 4. We chose a slit width of and the Grism14 with a wavelength ranging from 3200 Å to 7500 Å ([33]). The exposure time is 15 min. Reduction of the spectra was performed by using IRAF packages (IRAF is supported by the National Optical Astronomy Observatories (NOAO) in Tucson, Arizona http://iraf.noao.edu/iraf/web/irafhomepage.html), including bias subtraction, flatfielding, and cosmicray removal. Finally, the onedimensional spectrum was extracted. Using the winmk software (http://www.appstate.edu/~grayro/MK/winmk.htm), the normalized spectra are displayed in the upper pane of Figure 1. On the basis of the stellar spectral classification ([32]), we determined its spectral type to be G2V, which is similar with G5III obtained by [2].
2.2. Multicolor CCD Photometric Observations
HH Boo was monitored photometrically in 13 nights from December 25, 2010, to April 23, 2018, by using the next three telescopes: the 1.0 m telescope at Yunnan observatories (YNOs1m), the 60 cm telescope at Yunnan observatories (YNOs60cm), and the 85 cm telescope at Xinglong station of National Astronomical Observatories (Xinglong85cm) in China. The camera attached on Cassegrain focus of the 1.0 m telescope is DW436 CCD from Andor Technology, whose field is about . The CCD camera used on the 60 cm telescope is the same as that used on the 1.0 m telescope but has a larger field of view, about .
The first complete , , , and light curves were obtained during three nights on December 25, 27, and 30, 2010, with PI TE CCD camera mounted on the Xinglong 85 cm telescope. The effective field of view of the photometric system is at prime focus. The filter system was close to the standard JohnsonCousinBessel CCD photometric system [34]. The integration times were 40 s, 30 s, 20 s, and 10 s for , , , and bands, respectively. For each band, about 200 images were obtained ( = 197, = 196, = 196, and = 196). One image of the band is shown in Figure 2. GSC0347200043 and GSC0347201201 whose coordinates and magnitudes are listed in Table 1 were chosen as the comparison and the check star for HH Boo, respectively. The comparison and check stars are close enough to the variable that the range of airmass difference between both of them was very small. Therefore, an extinction correction was not made. The PHOT task (which measures magnitudes for a list of stars) in the IRAF aperture photometry package was used to reduce the observed images, including a flatfielding correction process.

The CCD photometric data obtained on December 25, 27, and 30, 2010, in , , , and bands are listed in Tables 2–5 and shown in Figure 3, respectively. The complete CCD light curves in the four bands with respect to the linear ephemeris,are shown in Figure 4. The magnitude differences between the comparison and the check stars are also shown in the low pane of this figure. The epoch in (2) is obtained by us (the mean value of four band light minima) and the period is from Grol et al. [31].




3. Orbital Period Investigation
The changes in the orbital period of HH Boo have been investigated by several authors (e.g., [16, 31]). Dal & Sipahi [16] found the period was decreasing at a rate of = d that could be explained by mass transfer or/and angular momentum loss via magnetic braking. Subsequently, Grol et al. [31] reported that the period of the binary showed a cyclic change by adding some eclipse times. However, the data did not cover the whole cycle of the curve. To investigate the period changes, we monitored the system since December 25, 2010. By using a leastsquares parabolic fitting method, 30 individual CCD times of light minimum were determined and listed in Table 6. On the other hand, Wide Angle Search for Planets (WASP) is an international consortium of several academic organisations performing an ultrawide angle search for exoplanets using transit photometry [35, 36]. WASP database (https://wasp.ceritsc.cz/) releases many photometric data of HH Boo obtained from 2004 to 2007. By using those data, 79 times of light minimum were obtained and are including in Table 7.


All available photoelectric and CCD times of light minimum were compiled and are shown in column 1 of Table 7. The values (observational times of light minimumcalculational times of light minimum) calculated by (2) are also listed in column 6 of Table 7 and plotted in the upper panel of Figure 5. The black solid dots in the figure refer to the data collected by Grol et al. [31], the blue solid dots refer to eclipse times computed with WASP data, the red solid dots refer to the data obtained by Nelson [21], Hubscher & Lehmann [22] or Hubscher [23], and green solid dots refer to the data observed by us. As displayed in the figure, the diagram may show a cyclic variation that could be explained by the light travel time effect via the presence of a third body (e.g., [37]). A leastsquares solution yields the following ephemeris: With this ephemeris, a cyclic oscillation with a period of = 6.58(11) yr and an amplitude of = 0.0018(1) d is determined. The residuals from (3) are showed in the lower panel of Figure 5 and listed in column 7 of Table 7.
4. Photometric Solutions
Photometric solutions of HH Boo were obtained by Dal & Sipahi [16] and Grol et al. [31]. However, those light curves were obtained with two small telescopes and were showing a little large scatter. As shown in Figure 4, the light curves are in higher precision and almost symmetric, which enables determining reliable photometric parameters. To understand its geometrical structure and evolutionary state, the , , , and light curves shown in Figure 4 were analyzed by using the WD code [38–41]. During the solution process, the effective temperature of star 1 was chosen as = 5680 K according to our result G2V and Grol et al. [31]. As shown in Figure 4, the depths of both minima are nearly the same indicating the nearly same temperature of both components. Therefore, we take the same values of the gravitydarkening coefficients and the bolometric albedo for both components, i.e., = = 0.32 [42] and = = 0.5 [43]. The limbdarkening coefficients were used according to Claret & Gimenez [44] (x and y are the bolometric and bandpass limbdarkening coefficients).
The adjustable parameters include the orbital inclination (); the mean temperature of star 2 (); the monochromatic luminosity of star 1 (, , , ); and the dimensionless potential of star 1 ( = , mode 3 for overcontact configuration). We chose the initial value of as 1.70 obtained by Grol et al. [31] and made it as an adjustable parameter. Then, we performed a differential correction until it converged and final solutions were derived. The solution converged at = 1.703(31). The photometric solutions are listed in column 2 Table 8 and the theoretical light curves computed with those photometric parameters are plotted in Figure 6. The light curves are nearly symmetric and no spotted solution is needed. The solution reveals that HH Boo is a Wtype shallowcontact binary with a degree of contact factor of = and a mass ratio of = 1.703(31). The geometrical structures at phases 0.0, 0.25, 0.5, and 0.75 are shown in Figure 7.

5. Discussions and Conclusions
Highprecision CCD light curves in , , , and bands obtained in 2010 are presented and were analyzed by the WD method. Our solution confirms that HH Boo is a Wtype overcontact binary system with a mass ratio of = 1.703(31). The temperature of the less massive component is about = 359 K higher than that of the more massive one. The fillout factor is about indicating that HH Boo is a shallowcontact binary system. The observational properties of HH Boo are similar to some EWtype contact binaries such as AE Phe [45], V524 Mon [46], NSVS 2669503 [47], DE Lyn [48], AQ Boo [49], GK Aqr [50], and V532 Mon [51]. All of them are Wtype shallowcontact systems with that are at the beginning phase of contact (e.g., [37]). They may be formed from detached EAs through a combination of Case A mass transfer and angular momentum loss via magnetic braking (e.g., [27, 52]), and the magnetic braking is more weaker than that of Pulsar (e.g., [53, 54]).
As shown in Figure 4, our light curves observed in 2010 are symmetric that may indicate weak photospheric activity in the system (e.g., [55, 56]). However, the light curves in , , and bands obtained in 2011 by Dal & Sipahi [16] show a positive O’Connell effect (see Figure 3 in their paper). The light curves published by Grol et al. [31] also show a positive O’Connell effect. These properties indicate that the light curve of HH Boo is variable. Our photometric solution reveals that HH Boo is a Wtype subclass binary, while Dal & Sipahi [16] demonstrated that HH Boo is most likely a member of the Atype subclass of W UMa binaries. The solutions obtained by Grol et al. [31] suggest the EWtype binary is a Wtype. They modeled the asymmetry of the maxima by using one cool star spot region located on the primary star. HH Boo is a solartype contact binary (Sp. = G2V) with a period of 0.318666 days where both components rotate very fast. The variation of the light curve may be caused by the magnetic activities.
To study the variations in the orbital period, we monitored the binary for about 8 years and 30 individual eclipse times were obtained. Moreover, by using WASP data, 79 times of light minimum were derived. Our analyses show that the diagram of HH Boo undergoes a cyclic oscillation. As shown in Figure 5, our data cover the whole cycle well. The cyclic oscillation could be more plausibly explained as the lighttravel time effect via the presence of a third body (e.g., [57, 58]). The period of the third body orbiting around the eclipsing pair of HH Boo is about 6.58 years. With the semiamplitude of the oscillation, the value is calculated to be 0.31(3) AU. Then, by using the following equation,a small mass function of = 0.00065(22) is determined. Combined with the masses of the two components = 0.627 , = 1.068 obtained by Grol et al. [31], the relations between the orbital inclination and the orbital radius and the mass of the assumed third body in HH Boo are shown in Figure 8. As shown in this figure, the minimal mass of the additional body is 0.129 indicating that it is an extremely cool star. By using the same method, some substellar objects orbiting evolved binaries were reported (e.g., [57, 59]). During the photometric solution, no third light is detected. This means that the orbital inclination of the third component could not be extremely lower. However, to further understand the properties of the variation of the light curves and the orbital periods, longterm photometric monitoring is needed.
Data Availability
All our data is already shown in the paper.
Disclosure
This paper makes use of data from the DR1 of the WASP data [60] as provided by the WASP consortium, and the computing and storage facilities at the CERIT Scientific Cloud, reg. no. CZ.1.05/3.2.00/08.0144 which is operated by Masaryk University, Czech Republic.
Conflicts of Interest
The authors declare that they have no conflicts of interest.
Acknowledgments
This project was supported by the Joint Research Fund in Astronomy (no. U1831109) under cooperative agreement between the National Natural Science Foundation of China (NSFC) and CAS and the Key Laboratory for the Structure and Evolution of Celestial Objects, CAS (no. OP201708). The lowresolution spectrograms were observed by Dr. Yuangui Yang using Xinglong 2.16 m telescope, and we would like to thank him. CCD photometric observations of HH Boo were obtained with Xinglong 85 cm telescope, the 1 m and the 60 cm RC reflect telescope, Yunnan Astronomical Observatory of CAS.
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Copyright © 2019 Jiajia He and Jingjing Wang. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.