Vol 17, No 8 (2017) / Qian

Physical properties and catalog of EW-type eclipsing binaries observed by LAMOST

Physical properties and catalog of EW-type eclipsing binaries observed by LAMOST

Qian Sheng-Bang1, 2, 3, 4, , He Jia-Jia1, 2, 3, Zhang Jia1, 2, 3, Zhu Li-Ying1, 2, 3, 4, Shi Xiang-Dong1, 2, 3, Zhao Er-Gang1, 2, 3, 4, Zhou Xiao1, 2, 3

Yunnan Observatories, Chinese Academy of Sciences, Kunming 650011, China
Key Laboratory of the Structure and Evolution of Celestial Objects, Chinese Academy of Sciences, Kunming 650216, China
Center for Astronomical Mega-Science, Chinese Academy of Sciences, Beijing 100012, China
University of the Chinese Academy of Sciences, Beijing 100049, China

† Corresponding author. E-mail: qsb@ynao.ac.cn


Abstract: Abstract

EW-type eclipsing binaries (hereafter called EWs) are strong interacting systems in which both component stars usually fill their critical Roche lobes and share a common envelope. Numerous EWs were discovered by several deep photometric surveys and there were about 40 785 EW-type binary systems listed in the international variable star index (VSX) by 2017 March 13. 7938 of them were observed with LAMOST by 2016 November 30 and their spectral types were identified. Stellar atmospheric parameters of 5363 EW-type binary stars were determined based on good spectroscopic observations. In the paper, those EWs are cataloged and their properties are analyzed. The distributions of orbital period (P), effective temperature (T), gravitational acceleration (log(g)), metallicity ([Fe/H]) and radial velocity (RV) are presented for these observed EW-type systems. It is shown that about 80.6% of sample stars have metallicity below zero, indicating that EW-type systems are old stellar populations. This is in agreement with the conclusion that EW binaries are formed from moderately close binaries through angular momentum loss via magnetic braking that takes a few hundred million to a few billion years. The unusually high metallicities of a few percent of EWs may be caused by contamination of material from the evolution of unseen neutron stars or black holes in the systems. The correlations between orbital period and effective temperature, gravitational acceleration and metallicity are presented and their scatters are mainly caused by (i) the presence of third bodies and (ii) sometimes wrongly determined periods. It is shown that some EWs contain evolved component stars and the physical properties of EWs mainly depend on their orbital periods. It is found that extremely short-period EWs may be older than their long-period cousins because they have lower metallicities. This reveals that they have a longer timescale of pre-contact evolution and their formation and evolution aremainly driven by angular momentum loss via magnetic braking.

Keywords: stars: binaries: close;stars: binaries: spectroscopic;stars: binaries: eclipsing;stars: evolution



1 Introduction

EW-type eclipsing binaries (hereafter called EWs) usually consist of two ellipsoidal FGK dwarfs that are in contact with each other and share a common convective envelope (CCE) that is lying between the inner and outer critical Roche-lobe surfaces. They areW Ursae Majoristype eclipsing variables with periods shorter than one day. Their light variation is continuous and it is impossible to specify the exact times of onset and end of eclipses. Their light amplitudes are usually < 0.8mag in V and the depths of the primary and secondary minima are almost equal (e.g., Samus et al. 2017 ). This indicates that the two components possess almost identical temperature and are in thermal contact in spite of different component masses. This kind of star system is different from EB-type binaries whose depths of primary and secondary minima are not equal and they are not in thermal contact. EW-type binaries are usually detected in older open clusters and globular clusters (e.g., Kaluzny & Rucinski 1993 ), but they are absent in young stellar clusters (e.g., Rucinski 1998 ). Based on these observational facts, some investigators assumed that EW-type binaries form from short-period detached binaries through angular momentum loss via magnetic braking (e.g., Guinan & Bradstreet 1988 ; Bradstreet & Guinan 1994 ) and will merge into rapidly rotating single stars (e. g., Zhu et al. 2016 ). The timescale of pre-contact evolution is from a few hundred million to a few billion years. However, how EWbinaries form is still unknown. It is possible that third bodies may play an important role in the origin of EWs by removing angular momentumfrom the central binary through early dynamical interaction and/or later evolution (e.g. Qian et al. 2014 , Zhu et al. 2013a ).

Thanks to several photometric surveys, such as the Catalina Sky Survey 1 (CSS, Drake et al. 2009 , 2014 ), the asteroid survey LINEAR 2 (Palaversa et al. 2013 ), All Sky Automated Survey 3 (ASAS, Pojmanski 1997 ; Pojmanski et al. 2005 ) and Northern Sky Variability Survey 4 (NSVS, Woźniak et al. 2004 ), a large number of EW binaries have been discovered. Since the data on variable stars including EWs are constantly being updated, the mission of VSX 5 (the international variable star index, Watson 2006 ) is to compile all of the new information together in a single data repository and provide tools necessary for the controlled and secure revision of the data. 40 785 EW-type binary systems were listed in VSX by 2017 March 13. Among the 40 785 EWs, the orbital periods of 40 646 systems were included. Those survey data are very useful for understanding the photometric properties of EW binaries. However, statistical spectroscopic properties of those sample stars are unclear because of the lack of spectral surveys. The light curves of many EW binaries have been observed recently, but their spectral types are generally unknown.

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, also called the Guo Shou Jing Telescope) is a special telescope with an effective aperture of about 4 meters that is located at Xinglong Station, administered by National Astronomical Observatories, Chinese Academy of Sciences. It has a field of view of 5 degrees and can simultaneously obtain the spectra of about 4000 stars with low-resolution of about 1800 in one exposure (Wang et al. 1996 ; Cui et al. 2012 ). The wavelength range of LAMOST is from 3700 to 9000 Å and is divided into two arms, i.e., a blue arm (3700–5900 Å) and a red arm (5700–9000 Å). The final spectrum of each target is obtained by merging several exposures. Huge amounts of spectroscopic data have been obtained (e.g., Zhao et al. 2012 ; Luo et al. 2012 , 2015 ).

In recent LAMOST data releases, about 19.5% of EW-type binaries (7938) in VSX were observed by the LAMOST survey from 2011 October 24 to 2016 November 30. Among the 7938 EWs, the orbital periods of 7930 samples are included in VSX. The distribution of the orbital period for EWs observed by LAMOST is shown in Figure 1. Also displayed in the figure is the period distribution of all EWs in VSX, in which 139 EWs without orbital periods are not shown. Those spectroscopic data can be used during the photometric solutions and the large amount of data representing stellar spectra from the LAMOST survey provide important information for studying EWs. In the paper, EW binaries observed in the LAMOST survey are cataloged. Then, based on the distributions of those atmospheric parame ters and some statistical correlations, the physical properties and the formation and evolutionary states of EW binaries are discussed.

Fig. 1 Period distribution of EWs. Open circles represent systems listed in the VSX catalog, while solid dots signify binaries observed by LAMOST. The peaks of the two period distributions are near 0.29 d (the dashed line).

2 Catalog of EWs Observed by LAMOST

In recent LAMOST data releases, about 7938 EWs in the VSX catalog were observed from 2011 October 24 to 2016 November 30 and their spectral types were obtained. Among the 7938 EWs, the stellar atmospheric parameters of 5363 systems were determined when their spectra had sufficiently high signal to noise. The stellar atmospheric parameters including the effective temperature T eff, gravitational acceleration log(g), metallicity [Fe/H] and radial velocity (RV) V r were automatically derived by the LAMOST stellar parameter pipeline when their spectra were of high quality (Wu et al. 2011b , 2014 ; Luo et al. 2015 ). Those stellar atmospheric parameters were determined based on the Universite de Lyon spectroscopic analysis software (ULySS) (Koleva et al. 2009 ; Wu et al. 2011a ). ULySS fits complete observed spectra by using a model spectrum that is generated by an interpolator by using the ELODIE library as a reference (e.g., Prugniel & Soubiran 2001 ; Prugniel et al. 2007 ). When T eff < 8000 K, the standard deviations are 110 K, 0.19 dex and 0.11 dex for T eff, log(g) and [Fe/H] respectively. For RV V r, the standard deviations are 4.91 km s−1 when T eff < 10 000 K (e.g., Gao et al. 2015 ).

The observations of 5363 EWs are cataloged in order of increasing VSX number. Some EWs were observed twice or more on different dates and we list all of the parameters. Values listed in Table 1 are the first 20 observations. The whole catalog is available through the internet (the electronic version of the catalog is at the website 6 ) and it will be improved by adding new data obtained by LAMOST in the future. The table includes names of the binaries, their right ascension (R.A.) and declination (Dec.) coordinates, types of light variation and orbital periods. These parameters are from the VSX catalog. Entries shown in Column (6) are the distances (in arcsec) between the two positions determined by the coordinates given in VSX and by LAMOST. These distances were used to identify EWs from the LAMOST samples based on the criterion Dist< 2″. The observing dates are listed in Column (7), while the determined spectral types of EWs are shown in Column (8). The stellar atmospheric parameters T eff, log(g), [Fe/H] and V r of the 5363 EWs are listed in Columns (9), (11), (13) and (15) respectively, and E 1, E 2, E 3 and E 4 in the table are their corresponding errors.

Name (1) R.A. (2) Dec. (3) Type (4) P (d) (5) Dist (6) Date (7) Sp. (8) T (K) (9) E 1 (10) log(g) (11) E 2 (12) [Fe/H] (13) E 3 (14) V r (km s−1) (15) E 4 (16)
HU And 003929.69 +400459.8 EW 0.285789 0.360 2011–10–28 G9 5158.97 246.09 4.431 0.352 0.126 0.229 0.73 18.92
IM And 004646.80 +393833.0 EW 0.270377 1.108 2014–12–12 K3 4764.13 208.42 4.182 0.299 −0.143 0.194 −136.85 15.35
IM And 004646.80 +393833.0 EW 0.270377 1.108 2015–09–13 K5 4977.45 73.29 4.692 0.105 −0.029 0.068 −157.15 5.28
LY And 022152.54 +383742.4 EW 0.34505 0.613 2013–11–12 G2 5751.81 29.16 4.090 0.041 −0.315 0.027 0.16 2.75
MT And 022704.78 +400328.5 EW 0.358781 0.767 2013–11–12 G3 5676.33 204.28 3.931 0.293 −0.136 0.190 14.41 15.32
MT And 022704.78 +400328.5 EW 0.358781 0.911 2013–10–14 G3 5751.14 25.60 4.144 0.035 0.005 0.024 −17.61 2.62
QX And 015757.78 +374822.5 EW 0.4121753 0.045 2014–11–10 F5 6501.66 2.24 4.174 0.001 −0.066 0.002 4.53 0.49
GK Aqr 221956.93 −007986.9 EW 0.3274145 0.344 2016–11–03 K0 5302.35 7.27 4.274 0.005 0.384 0.006 −17.35 1.81
GK Aqr 221956.93 −007986.9 EW 0.3274145 1.188 2012–10–04 K1 5365.27 56.54 4.368 0.080 0.458 0.053 −55.10 5.00
GM Aqr 222157.94 −028042.8 EW 0.3672853 0.491 2016–11–03 G7 5636.36 21.72 4.296 0.030 0.153 0.021 −66.66 2.33
GM Aqr 222157.94 −028042.8 EW 0.3672853 0.491 2012–10–04 G7 5715.99 132.99 4.308 0.190 0.109 0.124 34.44 10.36
GS Aqr 222733.63 −005757.6 EW 0.374067 1.671 2015–11–09 A5V 7023.10 3.25 4.276 0.002 −1.038 0.003 −53.16 0.81
GS Aqr 222733.63 −005757.6 EW 0.374067 0.479 2016–11–03 A7V 7027.32 7.89 4.323 0.010 −0.938 0.008 −112.34 1.25
AH Aur 062604.93 +275956.4 EW/KW 0.494106 0.267 2012–02–01 G3 6017.81 14.79 4.025 0.020 0.399 0.014 48.62 1.68
V0468 Aur 045611.66 +444638.9 EW 0.91278951 0.050 2012–02–09 F5 6548.18 182.53 3.856 0.262 −0.001 0.170 13.64 13.48
TU Boo 140458.04 +300001.5 EW/KW 0.3242868 0.144 2014–03–25 G3 5828.78 9.41 4.295 0.012 −0.027 0.009 7.13 1.32
TU Boo 140458.04 +300001.5 EW/KW 0.3242868 0.144 2016–05–18 G3 5710.93 9.32 4.165 0.012 −0.095 0.009 −13.39 1.24
TZ Boo 150809.13 +395812.9 EW/KW 0.297162 0.081 2016–02–25 G2 5597.28 6.21 4.179 0.008 −0.746 0.006 −59.65 0.92
AK Boo 133839.06 +241105.4 EW/KE 0.694030 0.436 2015–03–19 G7 5526.82 95.45 4.065 0.136 −0.326 0.089 45.21 7.77
AQ Boo 134726.86 +171824.7 EW 0.333139 0.936 2014–03–06 G0 5680.80 2.64 4.037 0.002 −0.466 0.002 −52.13 0.59

Table 1 Catalog of EWs Observed by LAMOST (the first 20 observations)

The temperatures of most EWs are lower than 8000K and their standard deviations could be estimated reliably. However, both components of EWs are rapidly rotating and highly deformed stars that share a common envelope. Do their spectra have sufficient tracers necessary for unique determination of atmospheric parameters? There are 25 EWs that were observed five times or more. To check the reliability of stellar atmospheric parameters and to answer the question, we determined the mean values of their atmospheric parameters and derived the corresponding standard errors.

The results are shown in Table 2 where their names and orbital periods are listed in the first and second columns respectively. The observational times are shown in the third column, while the average atmospheric parameters and their standard errors are displayed in the other columns. As shown in Table 2, standard errors of the effective temperatures for all targets are lower than 110K. Apart from two targets, the standard errors of the gravitational acceleration log(g) for the other targets are lower than 0.19 dex. The standard errors of the metallicity for most EWtargets are lower than 0.11 dex. These resultsmay indicate that it does nomatter if we use single star spectra to calibrate peanut-shaped stars and demonstrate that the extracted parameters could reach the mentioned level of precision. Moreover, because those EWs were observed at different phases, the associated results also reveal that there are no obvious effects of phase on the derived atmospheric parameters across multiple observations of the same object within errors.

Star name P (d) Times (K) Errors Errors Errors
T-Lyr1-15705 0.298933 8 5473.44 68.40 4.340 0.101 0.117 0.070
LINEAR 6102187 0.324026 7 5479.31 94.55 4.234 0.153 −0.047 0.041
CSS J074328.7+360725 0.391234 6 6158.67 67.58 4.157 0.072 −0.084 0.123
CSS J222851.7+065242 0.370323 5 6687.61 109.41 4.064 0.104 −0.322 0.089
CSS J111218.6+542629 0.317854 5 5421.22 61.99 4.339 0.072 −0.042 0.031
CSS J093510.3+313745 0.295414 5 6056.15 72.54 4.089 0.090 −0.572 0.017
CSS J090411.9+132908 0.273486 5 5135.15 33.33 4.165 0.090 −0.404 0.058
CSS J082217.4+064452 0.340438 5 5780.39 89.51 4.070 0.192 −0.236 0.093
CSS J073220.4+283612 0.274257 5 5330.87 47.72 4.332 0.113 −0.189 0.041
CSS J070315.8+423814 0.368082 5 5767.00 62.65 4.173 0.080 0.099 0.041
CSS J025901.9+313046 1.0544407 5 6304.31 46.60 3.754 0.091 0.148 0.068
CSS J025613.4+311517 0.538159 5 6237.42 46.37 4.106 0.020 −0.083 0.034
CSS J022913.6+042841 0.29825 5 5838.94 48.30 4.187 0.166 −0.755 0.189
CSS J011822.4+080543 0.454696 5 6119.61 56.82 4.131 0.124 −0.557 0.017
CSS J011310.1+370333 0.39251 5 6077.21 65.25 4.123 0.055 −0.177 0.051
LINEAR 6446092 0.30679788 5 5484.59 43.30 4.381 0.196 0.172 0.068
NSVS 4831297 0.369811 5 5926.45 24.09 4.202 0.086 0.202 0.053
KID 06964796 0.399961 5 5939.73 27.60 4.237 0.044 0.133 0.014
KID 11084782 0.586555 5 8222.08 17.16 4.011 0.018 −0.269 0.025
NSVS 4583537 0.40835410 5 6284.33 66.72 4.120 0.042 −0.111 0.049
VSX J065147.6+592649 0.4217 5 5996.10 35.18 4.127 0.079 0.411 0.018
NY Boo 0.32679 5 5779.52 30.98 4.115 0.055 −0.076 0.027
NSVS 7209962 0.295931 5 5276.37 37.39 4.273 0.080 0.348 0.055
OP Leo 0.391931 5 6104.86 51.49 4.129 0.061 −0.385 0.053
ASAS J081149–0111.1 0.567286 5 6275.02 93.33 4.346 0.030 0.143 0.034
V0449 Gem 0.270659 5 5045.36 42.53 4.321 0.049 −0.048 0.026
V1022 Tau 0.34717194 5 5882.69 52.00 4.193 0.049 −0.255 0.045

Table 2 Mean Atmospheric Parameters for 25 EWs Observed More than Four Times and Their Standard Errors

The relative distribution (the percentage of the number to the whole sample) of orbital period for the 5363 EWs is displayed in Figure 2. For comparison, the relative period distribution of all EWs in VSX is also shown in the figure. It is found that they nearly overlap. This indicates that the 5363 EWs could be used to represent the properties of all EWs in the complete VSX catalog. As shown in Figures 1 and 2, the period distribution peaks are near 0.29 d (dashed line). This is shorter than that given by Paczyński et al. ( 2006 ) who obtain a peak near 0.37 d based on ASAS data (e.g., Pojmanski 1997 ; Pojmanski et al. 2005 ). This may be caused by the fact that ASAS is dedicated to the detection of variability in bright stars, but many faint short-period EWs were discovered by recent deep photometric surveys (e.g., Drake et al. 2009 , 2014 ). Like cases analyzed by several investigators (e.g., Paczyński et al. 2006 ; Becker et al. 2011 ), it has a sharp cut-off at 0.2 d. A long tail exceeds 1 d and the tail actually extends to about 24 d.

Fig. 2 Relative distribution of orbital period for EWs. Open circles refer to all EWs listed in the VSX catalog, while solid dots are those 5363 EWs whose stellar atmospheric parameters were determined by using LAMOST data. The period distribution peaks are near 0.29 d (dashed line).

For some LAMOST spectra of EW-type binaries, their signal to noise values are not high enough to determine the stellar atmospheric parameters. In those cases, only spectral types were identified. The spectral types of those EWs are also cataloged in order of increasing VSX number. The entries shown in Table 3 are the first 20 spectral types in the catalog. The whole table is available at the website 7 via the internet. The catalog lists 3732 spectral types for 3055 EWs. Descriptions for those columns are the same as those in Table 1. For about 1691 EWs, only the spectral type was obtained by LAMOST. Both spectral types and stellar atmospheric parameters were determined for 5363 EWs. For the other ones, no results were obtained.

Name R.A. Dec. Type Period (d) Distance Date Sp.
MS And 022546.86 +395845.5 EW 0.777900 0.559 2013–11–12 K3
GS Aqr 222733.63 −005757.6 EW 0.374067 0.479 2012–10–29 A5V
MR Aur 055133.68 +310652.0 EW 0.690301 1.541 2011–11–10 A2IV
MR Aur 055133.68 +310652.0 EW 0.690301 1.541 2011–12–25 A1V
CK Boo 143503.76 +090649.4 EW/RS 0.355152 0.093 2016–04–24 F0
GH Boo 141451.51 +273415.7 EW 0.65951 0.833 2015–01–15 A8III
GH Boo 141451.51 +273415.7 EW 0.65951 0.053 2016–05–15 G7
GQ Boo 145936.67 +250244.9 EW 0.3846402 0.126 2012–06–05 G8
GW Cnc 084812.69 +210713.8 EW 0.281413 0.053 2015–03–08 G7
UZ CMi 075051.76 +033903.5 EW/DW 0.551361 0.180 2013–01–23 F6
BB CMi 075124.55 +045439.2 EW 0.792866160 0.204 2013–01–23 F0
SS Com 124939.08 +184211.9 EW/KW 0.412822 0.076 2016–05–19 G7
EY Com 131355.38 +310454.1 EW/KW 0.2993278 0.715 2012–02–26 K5
LP Com 123305.52 +270803.6 EW 0.33793358 0.107 2012–01–23 K4
LP Com 123305.52 +270803.6 EW 0.33793358 0.107 2012–01–11 K1
V2213 Cyg 192857.89 +430625.5 EW 0.350094 0.220 2014–09–13 G7
V2284 Cyg 192955.02 +485500.1 EW 0.306994 0.014 2013–09–14 G7
V2284 Cyg 192955.02 +485500.1 EW 0.306994 0.004 2015–10–01 G7
IV Dra 153623.26 +531911.3 EW 0.268105 0.017 2013–05–03 K5
IV Dra 153623.26 +531911.3 EW 0.268105 0.201 2012–06–17 K3

Table 3 Spectral Types Determined by LAMOST (the first 20 observations)

3 Distributions of Stellar Atmospheric Parameters for EWs

As aforementioned, the stellar atmospheric parameters of 5363 systems were determined and their relative period distribution is the same as that of all EWs in VSX. Therefore, they could be used to investigate the properties of all EWs. During the analyses, when the EWs were observed two times or more, the stellar atmospheric parameters, effective temperature , gravitational acceleration log(g) and metallicity [Fe/H] were averaged and we used the mean values. For RV V r, we did not average them because they were observed at different phases and vary with time.

The binary temperature distribution is shown in Figure 3 and the distribution has amain peak near 5700K (the solid line). This peak corresponds to the temperature of a G3-type main-sequence star with a stellar mass of about 0.97 (Cox 2000 ). This indicates that the majority of EWs are solar-type stars that have the protonproton (p-p) chain nuclear reaction occurring in their cores.

Fig. 3 Distribution of the effective temperature for EWs observed by LAMOST. Solid and dashed lines refer to the two peaks near 5700 K and 6600 K respectively.

Figure 3 also shows that there is a small peak near 6600K (the dashed line). This corresponds to the temperature of an F6-type main-sequence star with a stellar mass of about 1.35 . The transition from the first peak to the second small peak may reflect the central nuclear reaction changing from the p-p chain to the carbonnitrogen-oxygen (CNO) cycle. The distribution of gravitational acceleration log(g) is plotted in Figure 4. The distribution peaks near 4.16. It is apparent that the sample of EWsystems is homogeneous and most EWs are main-sequence binaries. This is in agreement with the idea that EWs are formed from detached main-sequence binaries via the combination of Case A mass transfer and angular momentum loss via magnetic braking (e.g., Qian et al. 2013a ).

Fig. 4 Distribution of gravitational acceleration log(g) for EWs observed by LAMOST. The dashed line refers to the peak near 4.16.

The metallicity ([Fe/H]) distribution is shown in Figure 5. EW-type binaries are usually composed of two solar-type stars. It is expected that their metallicities are similar to those detected among stars in the solar neighborhood (e.g., Rucinski et al. 2013 ). However, as visible in Figure 5, the metallicities of 80.6% of EWs are lower than that of the Sun, i.e., [Fe/H] < 0. For stars in the Galaxy, stellar metallicities are weakly correlated with their ages (e.g., Reid et al. 2007 ; Feltzing & Bensby 2009 ). The low metallicities indicate that most EWs are old stellar populations with longer ages. For a few percent of EWs, their metallicities are higher than 0.3 ([Fe/H] > 0.3). The possibility of unusually high metallicities may be through contamination by material from unseen degenerate objects (e.g., neutron stars or black holes) that are orbiting the binaries. Their progenitors were originally much more massive third stars in triple systems. Ten EWs that have the highest metallicities are shown in Table 4. They are a good source to search for potential degenerate objects (e.g., neutron stars or black holes) orbiting EWs.

Fig. 5 Distribution of metallicity [Fe/H] for EWs observed by LAMOST. It is apparent that most of the EWs have [Fe/H] < 0.
Name Period (d) Sp. T (K) log(g) [Fe/H] V r (km s−1)
CSS_J080956.0+131054 0.316903 K1 5692.04 4.491 0.677 −50.76
VSX J001137.3+303145 0.41222 F9 5719.4 4.088 0.637 −66.27
CSS_J083051.6+185801 0.331662 G5 5323.88 4.104 0.609 15.62
NSVS 2729390 0.347679 K0 5490.45 4.36 0.586 −63.81
ASAS J163229+0818.1 0.399559 G8 5815.14 4.254 0.56 −12.5
NSVS 4231740 0.40694612 F9/G5 5809.96 4.085 0.557 35.28
V1047 Her 0.32073733 K1 5466.2 4.211 0.545 −85.15
CSS_J032658.1+142940 0.385214 G7 5650.16 4.336 0.524 −2.23
CSS_J072417.0+224103 0.342338 G8/K1 5497.9 4.233 0.519 −37.3
CSS_J002629.5+445324 0.325968 K1/K3 5043.48 4.398 0.514 −0.99

Table 4 Ten EWs with the Highest Metallicities

The distribution of RV (V r) for these EWs is displayed in Figure 6. 7382 RVs for 5363 EWs are used for constructing the figure. A peak is near V r = −20 km s−1 and the distribution is symmetric. This may reveal that the V 0 of most EWs is close to this value. The amplitudes of RV curves for EWs are about 150–300 km s−1 (e.g., Rucinski et al. 2001 ). Figure 6 reflects a statistically random sampling of RV curves for EWs. Sixteen EWs with RVs larger than 200 km s−1 are shown in Table 5. They may be observed near the maxima or minima of the RV curves of those EWs.

Fig. 6 Distribution of RV V r for EWs observed by LAMOST. There is a peak near V r = −20 km s−1.
Name Period (d) Sp. T (K) log(g) [Fe/H] V r (km s−1)
UY Hya 0.7275 A2V 6697.05 4.233 −1.834 359.43
CSS_J212703.2+100332 0.452204 A5V 7065.8 4.27 −1.022 −321.77
CSS_J212703.2+100332 0.452204 A3V 6817.47 4.248 −1.039 −316.4
CSS_J005447.7+284030 0.314908 F0 6737.72 4.265 −0.888 −306.44
CSS_J032645.2+004931 0.705836 F5 5826.79 4.062 −1.405 −304.7
CSS_J112812.6+045202 0.316715 F5 5985.71 3.965 −0.881 301.29
CSS_J214828.6+090336 1.42465 F0 6611.71 4.365 −1.17 −285.31
CSS_J010448.7+373231 0.283694 F0 6139.34 4.047 −1.411 −276.72
CSS_J014525.5+360334 0.270282 G7 5342.15 4.101 −0.579 −264.58
CSS_J170410.6+274628 0.709314 A9V 6711.52 4.136 −0.627 −255.78
NSVS 7285749 0.323629 F0 6438 4.322 −1.018 242.96
CSS_J115356.5–022540 0.224256 K2 5074.82 4.745 −0.917 240.42
CSS_J072136.0+403327 0.340065 F4 6714.6 4.409 0.467 230.7
NSVS 7285749 0.323629 F0 6477.27 4.323 −0.963 222.12
CSS_J012955.1+391130 0.317905 F7 5932.42 4.1 −0.52 −216.98
LINEAR 14668373 0.224377 G8 4989.28 4.014 −1.591 −212.1
CSS_J032034.5+203607 0.320948 K5 5874.96 4.888 0.005 210.36

Table 5EWs Observed by LAMOST with RVs Larger than 200 km s−1 .

4 Statistical Correlations between Orbital Period and Stellar Atmospheric Parameters

The correlations between orbital period and effective temperature T eff, gravitational acceleration log(g) and metallicity [Fe/H] are shown in Figures 79, in which 55 EWs with orbital periods longer than 2 d are not displayed in the figures. Their orbital periods are from 2 d to 24 d. Six EWs are also not included in these figures because their orbital periods are unknown. Their atmospheric parameters are listed in Table 6. As shown in the three figures, the vast majority of EWs have periods in the range 0.2 < P < 0.6 d. The binary components in these short-period systems are usually main-sequence stars. However, there are some EW-type contact binaries with periods over 1 d. As plotted in Figure 8, the gravitational acceleration (log(g)) is weakly correlated with orbital period. The longer the orbital period is, the lower the gravitational acceleration will be. The lower log(g) of long-period systems indicates that the component stars have evolved from the zero-age main sequence.

Fig. 7 Correlation between orbital period and effective temperature for EWs observed by LAMOST. 55 EWs with orbital period longer than 2 d are not shown in the figure.
Fig. 8 The same as Fig. 7 but for the correlation between orbital period and gravitational acceleration.
Fig. 9 The same as Figs. 7 and 8 but for the correlation between orbital period and metallicity.
Name R.A. (deg) Dec. (deg) Date Sp. T (K) log(g) [Fe/H] V r (km s−1)
V0674 Per 54.115 36.37417 2014–11–13 K0 5046.24 4.307 −0.162 −39.99
V0726 Aur 80.69046 29.11806 2014–11–03 F9
NSV 3633 113.47608 48.00353 2013–11–22 F5 6327.91 4.174 −0.428 1.75
NSV 3633 113.47608 48.00353 2015–01–31 F5 6370.36 4.121 −0.348 4.84
NSV 3633 113.47608 48.00353 2016–02–19 F5 6353.80 4.128 −0.362 14.10
NSV 5580 185.6305 25.82833 2012–01–11 F7 6277.52 4.312 0.051 1.75
NSV 5580 185.6305 25.82833 2012–02–01 F7 6230.28 4.341 0.033 −17.33
NSV 5652 187.37508 29.51272 2012–01–11 F7 6227.60 4.121 0.063 −30.58
MG1 809127 240.96542 2.92944 2016–05–10 A5V
Konkoly V14 81.62869 12.95734 2013–10–02 F0 6857.67 3.474 0.396 −16.07
SvkV34 85.75447 40.95046 2012–02–18 K5 4520.01 4.382 −0.001 23.93

Table 6 LAMOST Observations of EWs without Orbital Periods

Figure 7 shows that there is a good correlation between the orbital period and effective temperature T eff for short-period EWs (e.g., P < 0.6 d). This is more clearly seen in Figure 10 where only short-period EWs are shown. The red dashed line in Figure 10 refers to the peak value of the period distribution. The relation is similar to the period-color relation for EWs (e.g., Eggen 1961 , 1967 ; Rucinski 1998 ; Terrell et al. 2012 ). Both main-sequence components in short-period EWs fill their critical Roche lobes and undergo CCE. It is expected that longerperiod systems should have higher-mass components with higher temperatures (e.g., Qian 2003 ). However, this relation shows a large scatter. This may be caused by the effect of the presence of third stars. Two examples of this case are 1SWASP J193127.17+465809.1 and 1SWASP J235935.22+362001.5 (e.g., Lohr et al. 2013 ). They are extremely short-period EWs with orbital periods 0.1976295 and 0.2016714d respectively. It is expected that they should be extremely cool binary systems. However, their spectral types determined by LAMOST are F0 with temperatures of 7027K and 7294K respectively. The possibility of a third body being present with spectral type of F0 can cause this difference. New spectroscopic and photometric data are very useful for studying these two interesting systems. Their positions in Figure 10 are shown as magenta solid triangles that greatly deviate from the general trend.

Fig. 10 Correlation between the orbital period and effective temperature for EWs with orbital periods shorter than 0.6 d. Magenta solid triangles refer to the positions of 1SWASP J193127.17+465809.1 and 1SWASP J235935.22+362001.5 that may contain bright third bodies. The green solid star represents the position of CSS_J080814.1+184933 with an inaccurate period, while the green solid circle to the right signifies the revised one.

The other possible cause for the large scatter in the period-temperature relation is that the periods of some binaries are inaccurate. One example is CSS_J080814.1+184933. Its orbital period given in VSX is 0.493868 d. The green solid star in Figure 10 refers to its position which does not follow the general trend of the period-temperature relation. The phased light curve from using this period is shown in Figure 11. As we see in the figure, there are two primary minima and two secondary minima in one phased light curve. This indicates that the period is inaccurate. By using those new data, the period of the binary was revised to 0.246746 d. The phased light curve with the revised period is displayed in Figure 12 which is a typical EW-type light curve. The green solid circle in Figure 10 represents the updated position of the binary with the revised period. As shown in Figure 10, the period-temperature relation is tight, but this relation is not linear.

Fig. 11 Phased light curve of CSS_J080814.1+184933 by using the orbital period (0.493868 d) given in VSX.
Fig. 12 The light curve of CSS_J080814.1+184933. The phases were computed with the revised period 0.246746 d.

Relations between the orbital period and log(g) and [Fe/H] for short-period EWs are shown in Figures 13 and 14 respectively. The positions of the three special EWs are also plotted in the two figures. Like what is depicted in Figure 10, the dashed lines represent the peak value of the period distribution. Figure 13 shows that log(g) is weakly correlated with orbital period. The relation for short-period systems (P < 0.29 d) is deeper than that for long-period ones (P > 0.29 d). As displayed in Figure 14, most of the EWs have lower metallicities (below the red solid line). Themetallicity is also weakly correlated with orbital period. Short-period EWs, with period shorter than 0.25 d, have metallicities below zero.

Fig. 13 The relation between orbital period (P) and gravitational acceleration log(g) for short-period EWs. Symbols are the same as those in Fig. 10.
Fig. 14 The relation between orbital period (P) and metallicity [Fe/H] for short-period EWs. Symbols are the same as those in Figs. 10 and 13.

5 Discussion and Conclusions

Numerous EWs have been discovered through several large photometric surveys (e.g., CSS, the asteroid survey LINEAR, ASAS and NSVS). However, spectroscopic data are lacking for those EWs. Among 40 785 EWs listed in the VSX catalog, 7938 were observed in the LAMOST spectral survey from 2011 October 24 to 2016 November 30. We catalog those EWs and their spectral types are given. We also present stellar atmospheric parameters for 5363 of them. By analyzing 25 EWs observed five times or more by LAMOST, we show that the standard errors of the effective temperatures, gravitational acceleration and metallicity are usually lower than 110K, 0.19 dex and 0.11 dex, respectively. These results may indicate that the spectra of EWs have sufficient tracers necessary for unique determination of atmospheric parameters and the extracted parameters could reach the mentioned level of precision. However, to check the results obtained by LAMOST, a careful selection and a detailed spectroscopic investigation of some EWs observed by LAMOST are needed. We are observing some EWs spectroscopically and will determine their stellar atmospheric parameters and compare them with those obtained by LAMOST.

The derived effective temperatures as well as the spectral types by LAMOST are very useful for generating the photometric solutions of their light curves. The other atmospheric parameters provide us with valuable information to understand the formation and evolution ary state of EWs. We found that the peak of the period distribution is near 0.29 d, which is shorter than that given by previous investigators (e.g., Paczyński et al. 2006 ). This indicates that a large number of short-period faint EWs were discovered recently with deep photometric surveys. The distributions of effective temperature (T), gravitational acceleration (log(g)), metallicity ([Fe/H]) and RV are presented for observed EWs. There are two peaks in the temperature distribution that correspond to p-p chain and CNO nuclear reactions respectively in the cores of the components. The distribution of gravitational acceleration log(g) indicates that the components of most EWs are main-sequence stars, which is consistent with the idea that EWs are formed from detachedmain-sequence binaries via a combination of Case A mass transfer and angular momentum loss via magnetic braking (e.g., Qian et al. 2013b ).

The detected metallicities of most sample stars (about 80.6%) are below zero. Since stellar metallicities are weakly correlated with their ages (e.g., Reid et al. 2007 ; Feltzing & Bensby 2009 ), this detection reveals that EW-type systems are old stellar populations. This supports the assumption that EWs need a long-term pre-contact evolution with timescales from a few hundred million to a few billion years. A few percent of EWs with unusually high metallicities may be contaminated by material from the evolutionary processes of unseen neutron stars or black holes in the systems. The progenitors of these unseen degenerate objects were originally much more massive third stars in triple systems. To date, neutron stars and black holes have usually been discovered in X-ray binaries through their X-ray radiation. They are formed through common envelope evolution and are influenced by their companion stars. If these unseen degenerate objects are confirmed, they will represent a new population of neutron stars and black holes (e.g., Qian et al. 2008 ; Ziolkowski 2010 ).

The correlations between orbital period and effective temperature, gravitational acceleration and metallicity are shown in previous sections. The scatters of those figures may be mainly caused by the presence of third bodies. EWs have the shortest period and lowest angular momentumamongmain-sequence binaries. It is assumed that they have a third body that plays an important role in their formation by removing angular momentum from the central binary (e.g., Qian et al. 2006 , 2007 , 2013a ; Zhu et al. 2013b ) For some systems, their orbital periods are inaccurate, which also causes scatters in those diagrams. It is shown that the relation between orbital period and effective temperature is tight but not linear.

Both gravitational acceleration and metallicity are weakly correlated with orbital period. The metallicities of all short-period EWs, with period shorter than 0.25 d, are below zero. These indicate that the physical properties of EWs mainly depend on their orbital periods. The formations and evolutionary states of EWs with different orbital periods may be quite different. This conclusion is supported by the relations between effective temperature (T) and gravitational acceleration log(g) and metallicity [Fe/H], which are shown in Figures 15 and 16 respectively. The dashed lines in the two figures refer to the peak of the temperature distribution at 5700K. It is found that both gravitational acceleration and metallicity are weakly correlated with effective temperature. Those extremely short-period EWs (P < 0.29 d; T < 5700K) usually have higher gravitational acceleration log(g) and lower metallicity [Fe/H]. They are main-sequence stars with little evolution and are older than their hotter cousins that have longer periods. This may suggest that they have a long timescale of pre-contact evolution and their formation and evolution are mainly driven by angular momentum loss via magnetic braking.

Fig. 15 The relation between effective temperature (T) and gravitational acceleration log(g) for EWs. The red dashed line represents the peak of the temperature distribution at 5700 K.
Fig. 16 The relation between effective temperature (T) and metallicity [Fe/H] for EWs. The red dashed line represents the peak of the temperature distribution.

References

Becker A. C. Bochanski J. J. Hawley S. L. et al. 2011 ApJ 731 17
Bradstreet D. H. Guinan E. F. 1994 Astronomical Society of the Pacific Conference Series 56 Interacting Binary Stars Shafter A. W. 228
Cox A.N. 2000 Introduction Cox A. N. Allen’s Astrophysical Quantities Cox A. N. New York AIP Press; Springer 1
Cui X.-Q. Zhao Y.-H. Chu Y.-Q. et al. 2012 RAA (Research in Astronomy and Astrophysics) 12 1197
Drake A. J. Djorgovski S. G. Mahabal A. et al. 2009 ApJ 696 870
Drake A. J. Graham M. J. Djorgovski S. G. et al. 2014 ApJS 213 9
Eggen O. J. 1961 Royal Obs. Bull. 31
Eggen O. J. 1967 MmRAS 70 111
Feltzing S. Bensby T. 2009 IAU Symposium 258 The Ages of Stars Mamajek E. E. Soderblom D. R. Wyse R. F. G. 23
Gao H. Zhang H.-W. Xiang M.-S. et al. 2015 RAA (Research in Astronomy and Astrophysics) 15 2204
Guinan E. F. Bradstreet D. H. 1988 NATO Advanced Science Institutes (ASI) Series C 241 Dupree A. K. Lago M. T. V. T. 345
Kaluzny J. Rucinski S. M. 1993 Astronomical Society of the Pacific Conference Series 53 Blue Stragglers Saffer R. A. 164
Koleva M. Prugniel P. Bouchard A. Wu Y. 2009 A&A 501 1269
Lohr M. E. Norton A. J. Kolb U. C. et al. 2013 A&A 549 A86
Luo A.-L. Zhang H.-T. Zhao Y.-H. et al. 2012 RAA (Research in Astronomy and Astrophysics) 12 1243
Luo A.-L. Zhao Y.-H. Zhao G. et al. 2015 RAA (Research in Astronomy and Astrophysics) 15 1095
Paczyński B. Szczygiel D. M. Pilecki B. Pojmański G. 2006 MNRAS 368 1311
Palaversa L. Ivezić Ž. Eyer L. et al. 2013 AJ 146 101
Pojmanski G. 1997 Acta Astronomica 47 467
Pojmanski G. Pilecki B. Szczygiel D. 2005 Acta Astronomica 55 275
Prugniel P. Soubiran C. 2001 A&A 369 1048
Prugniel P. Soubiran C. Koleva M. Le Borgne D. 2007 astro-ph/0703658
Qian S. 2003 MNRAS 342 1260
Qian S.-B. Liao W.-P. Fernández Lajús E. 2008 ApJ 687 466
Qian S.-B. Liu L. Kreiner J. M. 2006 New Astron. 12 117
Qian S.-B. Xiang F.-Y. Zhu L.-Y. et al. 2007 AJ 133 357
Qian S.-B. Zhang J. Wang J.-J. et al. 2013a ApJS 207 22
Qian S.-B. Liu N.-P. Li K. et al. 2013b ApJS 209 13
Qian S.-B. Wang J.-J. Zhu L.-Y. et al. 2014 ApJS 212 4
Reid I. N. Turner E. L. Turnbull M. C. Mountain M. Valenti J. A. 2007 ApJ 665 767
Rucinski S. M. 1998 AJ 116 2998
Rucinski S. M. Lu W. Mochnacki S. W. Ogloza W. Stachowski G. 2001 AJ 122 1974
Rucinski S. M. Pribulla T. Budaj J. 2013 AJ 146 70
Samus N. N. Kazarovets E. V. Durlevich O. V. Kireeva N. N. Pastukhova E.N. 2017 Astronomy Reports 61 80
Terrell D. Gross J. Cooney W. R. 2012 AJ 143 99
Wang S.-G. Su D.-Q. Chu Y.-Q. Cui X. Wang Y.-N. 1996 Appl. Opt. 35 5155
Watson C. L. 2006 Society for Astronomical Sciences Annual Symposium 25 47
Woźniak P. R. Vestrand W. T. Akerlof C. W. et al. 2004 AJ 127 2436
Wu Y. Singh H. P. Prugniel P. Gupta R. Koleva M. 2011a A&A 525 A71
Wu Y. Luo A.-L. Li H.-N. et al. 2011b RAA (Research in Astronomy and Astrophysics) 11 924
Wu Y. Du B. Luo A. Zhao Y. Yuan H. 2014 IAU Symposium 306 Statistical Challenges in 21st Century Cosmology Heavens A. Starck J.-L. Krone-Martins A. 340
Zhao G. Zhao Y.-H. Chu Y.-Q. Jing Y.-P. Deng L.-C. 2012 RAA (Research in Astronomy and Astrophysics) 12 723
Zhu L. Y. Qian S. B. Liu N. P. Liu L. Jiang L. Q. 2013a AJ 145 39
Zhu L.-Y. Qian S.-B. Zhou X. et al. 2013b AJ 146 28
Zhu L.-Y. Zhao E.-G. Zhou X. 2016 RAA (Research in Astronomy and Astrophysics) 16 68
Ziolkowski J. 2010 Mem. Soc. Astron. Italiana 81 294
Cite this article: Qian Sheng-Bang, He Jia-Jia, Zhang Jia, Zhu Li-Ying, Shi Xiang-Dong, Zhao Er-Gang, Zhou Xiao. Physical properties and catalog of EW-type eclipsing binaries observed by LAMOST. Cancer Biol Med. 2017; 8:087.

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