Vol 17, No 10 (2017) / Zhang

Radio stars observed in the LAMOST spectral survey

Radio stars observed in the LAMOST spectral survey

Zhang Li-Yun1, , Yue Qiang1, , Lu Hong-Peng1, , Han Xian-Ming L.1, 2, , Zhang Yong3, Shi Jian-Rong4, Wang Yue-Fei3, Hou Yong-Hui3, Zi-Huang Cao4

College of Physics/Department of Physics and Astronomy, Guizhou University, Guiyang 550025, China
Department of Physics and Astronomy, Butler University, Indianapolis, IN 46208, USA
Nanjing Institute of Astronomical Optics & Technology, National Astronomical Observatories, Chinese Academy of Sciences, Nanjing 210042, China
Key Laboratory of Optical Astronomy, National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012, China

† Corresponding author. E-mail: liy_zhang@hotmail.com


Abstract: Abstract

Radio stars have attracted astronomers’ attention for several decades. To better understand the physics behind stellar radio emissions, it is important to study their optical behaviors. The LAMOST survey provides a large database for researching stellar spectroscopic properties of radio stars. In this work, we concentrate on their spectroscopic properties and infer physical properties from their spectra, such as stellar activity and variability. We mined big data from the LAMOST spectral survey Data Release 2 (DR2), published on 2016 June 30, by cross-matching them with radio stars from FIRST and other surveys. We obtained 783 good stellar spectra with high signal to noise ratio for 659 stars. The criteria for selection were positional coincidence within 1.5′′ and LAMOST objects classified as stars. We calculated the equivalent widths (EWs) of the Ca ii H&K, Hδ, Hγ, Hβ, Hα and Ca ii IRT lines by integrating the line profiles. Using the EWs of the Hα line, we detected 147 active stellar spectra of 89 objects having emissions above the Hα continuum. There were also 36 objects with repeated spectra, 28 of which showed chromospheric activity variability. Furthermore, we found 14 radio stars emitting noticeably in the Ca ii IRT lines. The low value of the EW8542/EW8498 ratio for these 14 radio stars possibly alludes to chromospheric plage regions.

Keywords: stars;chromosphere - stars;activity - stars;radio - stars;spectroscopic



1 Introduction

Stars are, in general, studied by optical broad-band CCD observations, infrared photometry and spectroscopy as well as X-ray, ultraviolet and radio wavelengths (e.g., Butler et al. 2015 ; Osten et al. 2006 ). Late-type stars often display manifestations of magnetic activity such as chromospheric plages and flares, and coronal transition region emissions (e.g., Güdel 2002 ; Berdyugina 2005 ; Hall 2008 ). Radio emission from stellar sources was discovered over the past several decades (e.g., Güdel 2002 ). Most stellar objects have been identified in optical wavelengths, with radio stars contributing about 0.1 percent of the total stellar population (Flesch 2016 ). However, a wide variety of non-degenerate radio stars is present in the Hertzsprung-Russell diagram (e.g., Gudel 2002; Paredes 2005 ). Many different types of stars produce radio emissions (e.g., Wendker 1995 ; Güdel 2002 ). Among them there are pre-main sequence stars (Hughes 1988 ; Carkner et al. 1997 ), ultracool and brown dwarfs (Berger 2002 ; Berger et al. 2008 ), flare stars (White et al. 1989 ; Osten et al. 2006 ) and so on.

Wendker ( 1995 ) published a catalog of radio stars, which was then updated by Wendker ( 2015 ). It includes a total of 3699 stars or binary systems. 821 objects were detected at least once in meter to sub-millimeter wavelengths, and 2192 stars only have upper limits of radio flux. Some statistics on the variability of radio continuum emissions from stars were also provided (Wendker 1987 , 1995 ). The Faint Images of the Radio Sky at Twenty cm (FIRST) survey offers a factor of 50 improvement in observation of the radio sky using the NRAO Very Large Array (VLA) (e.g., Helfand et al. 1999 ). FIRST corresponds to about 10575 square degrees of sky coverage. Over most of the FIRST survey area, the detection limit was about 1 mJy. McMahon et al. ( 2002 ) published optical counterparts of 70 000 radio objects from the VLA FIRST radio survey. They also discussed the reliability of these identifications vs. optical and radio morphology. Ivezić et al. ( 2002 ) also associated 105 FIRST core sources with optical counterparts in the Sloan Digital Sky Survey (SDSS) and concluded that the spectra were extremely useful in examining the nature of the targeted sources. Kimball et al. ( 2009 ) chose 112 candidate radio stars from the FIRST and SDSS surveys, and analyzed their magnetic activities. Recently, the VLA FIRST project published a final catalog of 946432 objects and their identifications are at the FIRST survey’s website (Helfand et al. 2015 ).

Berger ( 2006 ) made radio observations of 90 dwarf stars and brown dwarfs of spectral types M5–T8 and discussed the distribution of magnetic field strengths. A few years later, Berger et al. ( 2008 ) obtained the first simultaneous radio, X-ray and Hα observations of ultra-cool dwarfs to study stellar magnetic activity and its relationship with chromospheric and coronal emissions. They analyzed about 100 late M and L dwarfs from VLA observations to explore the rotation-activity relationship (McLean et al. 2012 ). Flesch ( 2010 ) published a catalog by combining the radio and X-ray objects associated with optical objects. Later, they revised the catalog (Flesch & Hardcastle 2004 ).

Karoff et al. ( 2016 ) analyzed the LAMOST spectroscopic data of 5648 solar-like stars (including 48 superflare stars) and estimated the relationship between chromospheric activity and the occurrence of superflares. Frasca et al. ( 2016 ) studied LAMOST spectra of spectroscopic follow-up observations from the Kepler survey by means of the spectral subtraction of inactive templates using the code ROTFIT and determined the stellar parameters. Based on the chromospheric activity indicators Hα and Ca ii IRT lines, they found 442 chromospherically active stars and determined the relationship between these chromospheric fluxes and the precise rotation periods from Kepler photometry (Frasca et al. 2016 ).

The Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST) offers data to study chromospheric activity of radio stars. We present a catalog of radio stars observed in the LAMOST survey, and discuss properties of their chromospheric activity in the Hα, Hβ, Hγ, Hδ, Ca ii H & K and Ca ii IRT lines.

2 Data

LAMOST is a reflecting Schmidt optical telescope located at Xinglong Station of National Astronomical Observatories, Chinese Academy of Sciences (NAOC), and is designed to make stellar and extra-galactic spec-troscopic surveys. Its effective aperture is between 3.6 m and 4.9 m and the field of view is about 5° wide (Wang et al. 1996 ). It can obtain spectra of up to 4000 objects in a single exposure (e.g., Cui et al. 2012 ; Zhao et al. 2012 ; Luo et al. 2015 ). Specialists associated with LAMOST developed reliable automated methods and softwares (the LAMOST stellar parameter pipeline) to measure stellar fundamental parameters of the LAMOST spectro-scopic survey (the effective temperature, surface gravity and metallicity) (Wu et al. 2011 , 2014 ). The stellar spectra of 3.84 million LAMOST stellar objects were published on 2016 June 30 in the LAMOST Data Release 2 (DR2). These stellar spectra carry valuable information that can be used for the study of chromospheric activity and variability of radio stars. We cross-matched radio stars in the FIRST catalog and from other radio surveys (e.g., McMahon et al. 2002 ; Helfand et al. 2015 ; McLean et al. 2012 ; Wendker 1995 ; Flesch 2010 ) with the sub-sample of stellar sources in the LAMOST DR2 catalog, and obtained 783 stellar spectra with a signal to noise ratio (S/N) greater than about 8, which correspond to 659 individual targets.

In Table 1 we list these sources along with the LAMOST name (Col. (1)), observation date (Col. (2)), the S/N in Sloan ugriz bands (from Col. (3) to Col. (7)), spectral type (Col. (8)), magnitudes in ugriz JHK bands (Col. (9) to Col. (15)), and the references of radio stars (Col. (16)) in Table 1. We publish all the data in the online version of the journal.

LAMOST name Date S/N Sp Mtype Reference
(d) u g r i z Sp u g r i J H K
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
J030646.90+240209.1 2013-10-5 9.21 91.4 187.19 263.4 212.76 G5 99 12.47 11.48 0 9.83 9.26 9.13 Wendker 1995
J032858.10+311803.6 2013-12-6 2.16 10.79 28.78 72.38 70.6 M1 99 19.06 16.92 15.65 99 99 99 Wendker 1995
J034743.11+241655.5 2013-1-2 27.72 113.05 194.54 247.8 197.03 F5 99 12.19 11.66 11.28 10.41 10.09 10 Wendker 1995
J040313.95+255259.7 2014-1-30 4.83 36.88 73.77 100.64 77.58 K3 99 99 99 99 10.84 10.32 10.16 Wendker 1995
J040313.95+255259.7 2014-1-30 5.69 42.49 86.64 118.82 93.38 K3 99 99 99 99 10.84 10.32 10.16 Wendker 1995
J040439.37+215818.4 2013-10-26 3.09 26.61 96.13 223.29 233.24 M3 99 15.61 14.22 0 99 99 99 Wendker 1995
J040530.88+215110.5 2013-10-26 2.83 24.49 82.83 178.17 183.59 M1 99 15.66 14.18 12.88 99 99 99 Wendker 1995
J040530.87+215110.6 2013-9-27 1.08 3.01 10.29 30.29 36.76 M3 99 99 99 99 10.95 10.29 10.06 Wendker 1995
J041314.16+281910.4 2014-1-30 1.26 16.33 51.06 129.78 133.71 M3 99 14.3 13.14 0 9.64 8.87 8.63 Wendker 1995
J041314.16+281910.4 2014-1-30 1.39 13.35 42.11 108.85 110.71 M3 99 14.3 13.14 0 9.64 8.87 8.63 Wendker 1995
J041314.14+281910.8 2012-1-22 5.89 72.59 77.73 160.46 158.39 M4 99 99 99 16.8 99 99 99 Wendker 1995
J041414.60+282757.8 2014-1-25 2.44 32.1 95.58 219.91 233.05 M4 99 14.84 13.41 0 9.47 8.67 8.19 Wendker 1995
J041414.60+282757.8 2014-1-25 2.43 32.5 83.64 195.26 208.65 M4 99 14.84 13.41 0 9.47 8.67 8.19 Wendker 1995
J041414.60+282757.8 2014-1-30 1.26 9.27 33.71 94.72 100.07 M4 99 14.84 13.41 0 9.47 8.67 8.19 Wendker 1995
J041417.01+281057.5 2014-1-25 16.93 65.53 138.83 197.81 172.4 K3 99 14.57 13.42 11.41 9.56 8.24 7.13 Wendker 1995
J041417.01+281057.5 2014-1-25 14.47 61.34 146.25 210.96 188.6 K3 99 14.57 13.42 11.41 9.56 8.24 7.13 Wendker 1995
J041430.60+285129.8 2014-1-25 1.17 13.18 36.51 60.38 52.18 M0 99 99 99 99 9.21 8.6 8.36 Wendker 1995
J041447.31+264626.1 2014-1-30 2.16 22.26 78.1 159.77 146.51 M3 99 14.72 13.23 0 9.9 9.18 8.87 Wendker 1995
J041447.31+264626.1 2014-1-30 2.26 20.72 71.63 146.83 134.95 M3 99 14.72 13.23 0 9.9 9.18 8.87 Wendker 1995
J041449.29+281230.2 2013-2-8 1.42 10.5 42.98 138.73 162.08 M4 99 15.81 14.21 0 99 99 99 Wendker 1995
J041449.28+281230.5 2014-1-30 1.9 6.39 29.15 101.29 122.72 M4 99 99 99 99 9.65 8.57 8.12 Wendker 1995
J041449.29+281230.2 2012-1-4 2.47 4.63 16.99 70.43 121.46 M4 99 15.81 14.21 0 99 99 99 Wendker 1995
J041449.28+281230.5 2014-1-25 2.52 7.05 30.77 84.54 97.12 M4 99 99 99 99 9.65 8.57 8.12 Wendker 1995
J041449.29+281230.2 2011-12-18 1.43 11.03 32.67 110.91 132.25 M4 99 15.81 14.21 0 99 99 99 Wendker 1995
J041449.28+281230.5 2014-1-30 2.13 5.36 21.33 78.63 97.81 M4 99 99 99 99 9.65 8.57 8.12 Wendker 1995
J041449.28+281230.5 2014-1-25 2.15 5.4 22.98 62.98 74.2 M4 99 99 99 99 9.65 8.57 8.12 Wendker 1995
J041449.29+281230.2 2012-1-13 2.33 12.65 40.9 135.12 160.99 M4 99 15.81 14.21 0 99 99 99 Wendker 1995
J041447.87+264810.7 2014-1-30 3.04 30.09 96.35 187.72 169.3 M1 99 14.44 13.02 0 9.87 9.05 8.81 Wendker 1995
J041447.87+264810.7 2014-1-30 3.14 31.68 105.94 204.66 183.36 M1 99 14.44 13.02 0 9.87 9.05 8.81 Wendker 1995
J041628.11+280735.0 2014-1-30 2.57 46.41 75.74 135.1 123.85 M0 99 0 12.57 11.62 9.25 8.52 8.32 Wendker 1995
J041628.11+280735.0 2014-1-25 5.47 49.99 113.28 170.27 148.13 M0 99 0 12.57 11.62 9.25 8.52 8.32 Wendker 1995
J041738.94+283300.1 2014-1-25 2.21 36.92 82.02 170.67 156.32 M1 99 14.3 12.97 0 9.98 9.29 9.05 Wendker 1995
J041738.94+283300.1 2014-1-25 2.05 33.01 75 147.49 134.55 M1 99 14.3 12.97 0 9.98 9.29 9.05 Wendker 1995
J041831.12+281628.4 2014-1-30 1.68 3.63 6.89 21.27 25.69 M5 99 15.97 13.81 0 9.83 8.68 7.88 Wendker 1995
J041831.12+281628.4 2014-1-25 7.43 20.37 48.19 84.4 85.23 M6 99 15.97 13.81 0 9.83 8.68 7.88 Wendker 1995
J041831.58+281658.5 2014-1-30 1.31 4.26 9.59 30.76 34.72 M4 99 99 99 99 10.52 9.77 9.36 Wendker 1995
J041831.60+281658.3 2013-2-8 1.18 4.68 19.67 59.07 66.4 M3 99 99 99 14.6 99 99 99 Wendker 1995
J041831.60+281658.3 2012-1-4 1.86 6.28 13.43 31.11 47.75 M4 99 99 99 14.6 99 99 99 Wendker 1995
J041912.81+282933.0 2014-1-25 1.73 3.53 8.07 24.69 28.43 M4 99 99 99 99 10.49 9.7 9.31 Wendker 1995
J041912.80+282932.7 2013-12-31 3.1 19.7 62.99 172.79 186.95 M4 99 15.94 14.81 0 99 99 99 Wendker 1995
J041912.80+282932.7 2012-10-5 1.11 1.91 4.09 33.3 62.5 M4 99 15.94 14.81 0 99 99 99 Wendker 1995
J041915.83+290626.9 2014-1-25 1.35 4.75 5.91 16.34 14.93 M0 99 99 99 99 9.1 8.22 7.74 Wendker 1995
J041926.27+282613.9 2014-1-25 2.34 31.66 80.64 121.8 104.47 K7 99 13.75 12.24 0 9.5 8.65 8.42 Wendker 1995
J041935.45+282721.8 2014-1-25 1.35 1.84 2.66 8.42 11.1 M6 99 99 99 99 10.95 10.37 9.97 Wendker 1995
J041935.46+282721.3 2012-10-5 1.74 2.91 3.63 16.39 30.93 M5 99 99 99 14 99 99 99 Wendker 1995
J041935.46+282721.3 2013-12-31 2.57 13.17 44.03 161.5 185.74 M6 99 99 99 14 99 99 99 Wendker 1995

(1) We only show some part of the radio stars in Table 1. The parameters of all the radio stars we studied are available in http://www.raajournal.org/docs/Supp/ms20170016Table1.txt. (2) The code 99 means no data. The magnitudes in different bands are from the SDSS (Gunn et al. 1998 ; York et al. 2000 ) and 2MASS catalogs (Skrutskie et al. 2006 ).

Table 1 Radio Stars Observed with LAMOST

3 Spectroscopic analyses

We calculated the equivalent widths (EWs) of the spectral lines using the usual formula where Fλ is the line flux and F C is that at the continuum. With this definition, emission lines have a positive EW. The spectral regions for evaluating the continuum at the two sides of each line are 6555–6560 Å and 6570–6575 Å for Hα; 4840.0–4850.0 Å and 4875.04885.0 Å for Hβ, 4310.0–4330.0 Å and 4350.0–4370.0 Å for Hγ 4075.0–4095.0 Å and 4110.0–4130.0 Å for Hδ; and 3952.8–3956.0 Å and 3974.0–3976.0 Å for Ca ii K, respectively. These wavelength intervals are close to those by Hilton et al. ( 2010 ) and West et al. ( 2004 , 2011 ) for the analyses of SDSS spectra. To select radio stars with a high activity level, we used the criteria for the Ha line similar to those of West et al. ( 2011 ) and Yi et al. ( 2014 ). The EWs of the Hα line are larger than 0 Å for OB AFGK stars (for M stars, they are larger than 0.75 A), and are simultaneously larger than their uncertainties, whereas the height of the Hα emission must be at least 3 times its noise (e.g., Hawley et al. 2002 ; Zhang et al. 2016 ).

We also visually inspected all candidates for active radio stars and manually checked their chromospheric activity spectral lines. The uncertainties in spectral types of the LAMOST observations arise from two subtypes (e.g., Yi et al. 2014 ; Zhang et al. 2016 ) with respect to the model of a standard star. We normalized the LAMOST spectra of OBAFGK stars to their continuum by a polynomial fit using the continuum package in the IRAF software 1 . We plotted the LAMOST observed (left panels) and their normalized continuum spectra (right panels) in Figure 1.

Fig. 1 Examples of LAMOST spectra (left) and their continuum normalized spectra (right) for radio stars. Some of them show obvious emissions in the Ca ii H&K, Hi, H7, H,3, Ha and Ca ii IRT lines.

We report the LAMOST names of radio stars observed in the LAMOST spectral survey below the panels in Figure 1. At the top of the panels, there is the spectroscopic name from the LAMOST survey, for exampie, spec-56683-VB063N29Vl-sp07-206.fits. We also marked the spectral types of the radio stars and the different chromospheric activity indicators in the figure. We calculated the EWs by integrating over the emission profile using the SPLOT task in the IRAF package. The methods for calculating the EWs and their uncertainties were similar to those of Zhang & Gu ( 2008 ). Using the EWs of the Ha line, we detected 147 active spectra of 89 objects with emissions above the continuum. These are listed in Table 2.

No. Name Sp Re Name 6cm kHz Flux V1 Hα V2 Hβ Hγ Hδ Ca iiH Ca iiK
(1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16)
1 J032858.10+311803.6 M2 [1] 107HH 12 230 38.0 - 11.827 ± −0.223 - 6.245 ± 0.201 4.200 ± 0.131 ± ± ±
2 J040439.37+215818.4 M3 [1] HBC 360 0.6 - 5.450 ± 0.228 - 6.673 ± 0.221 5.019 ± 0.071 5.386 ± 0.376 16.289 ± 0.06 14.425 ± 0.365
3 J040530.88+215110.5 M3 [1] HBC 362 0.6 - 6.931 ± 0.171 v 7.676 ± 0.019 6.305 ± 0.367 6.766 ± 0.557 20.438 ± 0.19 22.120 ± 1.15
3 J040530.87+215110.6 M2 [1] HBC 362 0.6 - 12.186 ± 0.774 v 9.359 ± 0.449 10.360 ± 7.417 ± 38.379 ± 2.36 ±
4 J041314.16+281910.4 M2 [1] HBC 365 0.6 - 2.116 ± 0.16 v 1.341 ± 0.118 ± ± 9.878 ± 0.272 9.981 ± 0.729
4 J041314.16+281910.4 M3 [1] HBC 365 0.6 - 3.133 ± 0.162 v 2.615 ± 0.078 1.208 ± 0.042 ± 10.916 ± 0.384 19.218 ± 0.208
4 J041314.14+281910.8 M3 [1] HBC 365 0.6 - 3.265 ± 0.338 v 2.472 ± 0.037 0.879 ± 0.04 ± 15.317 ± 1.42 18.765 ± 1.505
5 J041414.60+282757.8 M3 [1] FN Tau 75.0 230 31.0 - 18.444 ± 0.646 v 11.906 ± 0.016 9.399 ± 0.082 9.072 ± 0.072 27.762 ± 1.79 28.230 ± 0.32
5 J041414.60+282757.8 M4 [1] FN Tau 75.0 230 31.0 - 17.433 ± 0.397 v 9.099 ± 0.207 8.102 ± 0.081 ± ± ±
5 J041414.60+282757.8 M3 [1] FN Tau 75.0 230 31.0 - 13.844 ± 0.316 v 8.683 ± 0.15 8.013 ± 0.123 7.658 ± 0.064 23.080 ± 2.46 18.306 ± 1.244
6 J041417.01+281057.5 K5 [1] CW Tau 0.30 230 96.0 - 222.945 ± 1.855 v 41.770 ± 2.61 25.284 ± 3.496 18.667 ± 0.057 54.338 ± 8.142 76.067 ± 1.617
6 J041417.01+281057.5 K5 [1] CW Tau 0.30 230 96.0 - 87.379 ± 6.671 v 12.457 ± 1.153 ± ± ± ±
6 J041417.01+281057.5 K5 [1] CW Tau 0.30 230 96.0 - 247.056 ± 5.244 v 44.394 ± 1.566 18.490 ± 1.65 11.051 ± 0.249 39.122 ± 4.298 48.512 ± 2.378
6 J041417.01+281057.5 K5 [1] CW Tau 0.30 230 96.0 - 80.762 ± 1.228 v 12.604 ± 1.056 ± ± 8.097 ± 0.58 19.073 ± 0.537
7 J041430.60+285129.8 K7 [1] LkCa 2 0.6 - 1.433 ± 0.074 - ± ± ± ± ±
8 J041447.31+264626.1 M3 [1] FP Tau 230 50.0 - 46.160 ± 0.96 v 14.838 ± 0.278 15.869 ± 2.119 12.347 ± 0.127 19.700 ± 1.23 8.361 ± 0.061
8 J041447.31+264626.1 M2 [1] FP Tau 230 50.0 - 54.556 ± 3.846 v 13.228 ± 0.048 14.698 ± 1.798 17.314 ± 1.614 26.452 ± 0.902 10.395 ± 0.045
9 J041449.29+281230.2 M2 [1] FO Tau 230 50.0 - 114.281 ± 3.481 v 55.119 ± 2.231 43.856 ± 3.626 41.780 ± 1.17 94.798 ± 2.21 84.371 ± 1.429
9 J041449.28+281230.5 M4 [1] FO Tau 230 50.0 - 121.209 ± 4.291 v 79.030 ± 3.58 31.071 ± 1.029 23.052 ± 0.152 25.238 ± 0.392 18.452 ± 1.328
9 J041449.29+281230.2 M5 [1] FO Tau 230 50.0 - 135.805 ± 14.695 v 170.04 ± 33.44 282.39 ± 51.99 101.98 ± 41.33 234.33 ± 167.9 167.28 ± 53.53
9 J041449.28+281230.5 M4 [1] FO Tau 230 50.0 - 127.529 ± 8.071 v 72.666 ± 4.094 49.071 ± 0.299 37.048 ± 0.748 54.500 ± 2.66 53.433 ± 1.343
9 J041449.29+281230.2 M4 [1] FO Tau 230 50.0 - 130.805 ± 1.595 v 66.829 ± 4.209 53.685 ± 4.805 55.888 ± 7.488 100.228 ± 1.58 93.888 ± 26.912
9 J041449.28+281230.5 M4 [1] FO Tau 230 50.0 - 132.381 ± 1.281 v 67.971 ± 0.029 25.734 ± 0.596 19.374 ± 0.186 21.534 ± 0.896 15.899 ± 0.821
9 J041449.28+281230.5 M3 [1] FO Tau 230 50.0 - 120.660 ± 0.76 v 72.939 ± 3.399 48.004 ± 0.314 36.778 ± 1.218 43.150 ± 38.948 41.548 ± 4.222
9 J041449.29+281230.2 M4 [1] FO Tau 230 50.0 - 113.665 ± 5.865 v 69.514 ± 2.924 51.335 ± 2.565 45.608 ± 0.342 87.798 ± 9.53 65.534 ± 3.604
10 J041447.87+264810.7 M2 [1] CX Tau 230 40.0 - 34.289 ± 1.079 v 14.527 ± 0.467 14.483 ± 1.653 1.234 ± 11.846 21.396 ± 0.214 7.219 ± 0.83
10 J041447.87+264810.7 M2 [1] CX Tau 230 40.0 - 27.862 ± 0.998 v 15.487 ± 0.897 17.394 ± 3.324 ± 16.131 ± 1.079 7.303 ± 0.76
11 J041628.11+280735.0 K7 [1] DM+10551 0.6 - 4.108 ± 0.08 v 1.483 ± 0.138 ± ± 11.007 ± 0.283 12.597 ± 0.323
11 J041628.11+280735.0 K5 [1] DM+10551 0.6 - 4.836 ± 0.035 v 1.934 ± 0.04 ± ± 13.657 ± 1.15 15.203 ± 0.273
12 J041738.94+283300.1 M2 [1] HBC 371 0.5 - 3.772 ± 0.178 n 2.349 ± 0.089 1.643 ± 0.114 1.658 ± 0.153 12.747 ± 0.333 16.889 ± 1.131
12 J041738.94+283300.1 M2 [1] HBC 371 0.5 - 3.860 ± 0.042 n 2.374 ± 0.172 ± ± 16.701 ± 0.499 18.438 ± 1.552
13 J041831.12+281628.4 M3 [1] DD Tau 0.5 230 50.0 - 300.448 ± 15.252 v 43.136 ± 0.406 27.009 ± 3.429 13.576 ± 1.236 13.695 ± 0.485 ±
13 J041831.12+281628.4 M2 [1] DD Tau 0.5 230 50.0 - 264.300 ± 17.1 v 54.286 ± 5.596 34.478 ± 6.778 23.456 ± 0.256 44.289 ± 7.049 29.528 ± 0.802
14 J041831.58+281658.5 M3 [1] CZ Tau 0.5 - 4.217 ± 0.142 v ± ± ± ± ±
14 J041831.60+281658.3 M3 [1] CZ Tau 0.5 - 5.021 ± 0.085 v ± ± ± ± ±
14 J041831.60+281658.3 M4 [1] CZ Tau 0.5 - 6.048 ± 0.004 v 3.867 ± 0.345 4.725 ± 0.108 ± 10.755 ± 0.22 17.511 ± 0.909
15 J041912.81+282933.0 M3 [1] FQ Tau 230 40.0 - 5.343 ± 0.073 v 55.969 ± 3.319 23.100 ± 0.23 9.380 ± 1.99 57.464 ± 5.59 53.208 ± 5.358
15 J041912.80+282932.7 M4 [1] FQ Tau 230 40.0 - 51.128 ± 1.292 v 31.753 ± 1.003 19.347 ± 0.193 17.039 ± 0.031 31.210 ± 0.89 31.312 ± 0.528
15 J041912.80+282932.7 M6 [1] FQ Tau 230 40.0 - 77.950 ± 6.14 v 86.059 ± 4.419 55.317 ± 2.843 100.847 ± 7.153 177.328 ± 60.08 67.410 ± 9.37
16 J041915.83+290626.9 M0 [1] BP Tau 15 9.0 - 135.743 ± 5.157 v 26.127 ± 1.947 16.389 ± 0.329 ± ± ±
16 J041915.83+290626.9 M1 [1] BP Tau 15 9.0 - 124.073 ± 9.173 v 23.195 ± 0.425 19.951 ± 2.871 ± ± ±
17 J041926.27+282613.9 M0 [1] V819 Tau 230 34.0 v 3.367 ± 0.022 - ± ± ± ± ±
18 J041935.46+282721.4 M6 [1] FR Tau 230 50.0 - 139.111 ± 7.411 v 34.176 ± 0.436 21.649 ± 0.641 17.210 ± 0.8 19.022 ± 3.74 10.355 ± 0.225
18 J041935.45+282721.8 M5 [1] FR Tau 230 50.0 - 32.268 ± 0.932 v ± ± ± ± ±
18 J041935.46+282721.3 M6 [1] FR Tau 230 50.0 - 77.453 ± 2.067 v 10.83 ± 0.31 12.307 ± 0.967 ± ± ±
18 J041935.46+282721.3 M6 [1] FR Tau 230 50.0 - 67.614 ± 4.196 v 66.730 ± 2.18 58.261 ± 1.409 36.353 ± 0.137 73.331 ± 5.04 52.204 ± 0.066
19 J042155.63+275506.0 M0 [1] DE Tau 0.58 - 80.984 ± 1.344 - 44.363 ± 1.567 26.962 ± 1.328 32.234 ± 3.176 102.723 ± 11.33 93.660 ± 3.88

Notes: [1] Wendker 1995 ; [2] Helfand et al. 2015 ; [3] McLean et al. 2012 ; [4] Berger et al. 2008 ; [5] Helfand et al. 1999 ; [6] McMahon et al. 2002 ; [7] Flesch 2010 ; [8] Helfand et al. 2015 We only show some part of the radio stars in Table 2. No value with ± means there is no data point. The parameters of all radio stars are available in http://www.raa-journal.org/docs/Supp/ms20170016Table2.txt.

Table 2 EWs of Chromospheric Emission Lines for 89 Radio Stars

In this table we quote the number (Col. (1)), the LAMOST name (Col. (2)), spectral type (Col. (3)), the sources of radio stars (Col. (4)) other name (Col. (5)), radio flux of 6 cm and other radio wavelengths (Col. (6) to Col. (8)), the variation in the radio wavelength (Col. (9)), EWs of Hα (Col. (10)), the flag for variation of chromospheric activity (Col. (11)), Hβ, Hγ, Hδ and Ca ii H&K lines (Col. (12) to Col. (16)). The printed version of Table 2 displays only a few lines as examples, but the full data set in Table 2 is available in the online version of the journal and can be downloaded as an electronic table from the on-line database at http://www.raa-journal.org/docs/Supp/ms20170016Table2.txt.

4 Discussion and Conclusions

We studied the chromospheric activity of radio stars using different chromospheric activity indicators, and discussed the statistical properties of chromospheric variability.

4.1 Chromospheric Activity Emission and Variability

Using the EWs of the Hα line, we detected 147 active spectra of 89 objects with emissions above the Hα continuum. There are 34 objects with repeated LAMOST observations. Many of our objects are T Tauri (T Tau) stars in the Tau-Aur complex (e.g., Fernandez et al. 1995 ; Kenyon & Hartmann 1995 ; Mohanty et al. 2005 ; Nguyen et al. 2012 ). The spectral characteristics (the emission of Hα lines and other spectral lines) of many of the T Tau stars that correspond to our LAMOST objects are consistent with previous low or high-resolution spectra in the literature (see, e.g., Fernandez et al. 1995 ; Kenyon & Hartmann 1995 ). The EWs in Ha are also similar to previous results. It is interesting to note that the criteria that we adopted could be useful for selecting new low-mass T Tau candidates in LAMOST and SDSS surveys in the future (Luhman et al. 2017 ). Information on the variation of radio and chromospheric activity is listed in Col. (9) and Col. (11) of Table 2, where v represents the variation of radio flux (‘n’ means that there is no variation and ‘−’ means that there is no data point). We regarded the peak-to-peak EW variation as an index of Hα variability. Whenever it is larger than 3 times the maximum error, we consider the object as variable and put a flag “v” in Col. (11) of Table 2 (Zhang et al. 2016 ). Among these stars, 28 of them show chromospheric activity variability. We plotted the LAMOST observed spectra (left) and continuum normalized spectra (right) of active objects with repeated observations and variation of activity in Figure 2. Different colors represent spectra of the same object acquired at different times. We also plotted some examples of the observed spectra and their EW light curves in the Hα, Hβ, Hγ, Hδ, Ca ii H&K and Ca ii IRT lines in Figure 3.

Fig. 2 Examples of sources with multiple LAMOST spectra. The observed spectra are shown in the left panels; the continuum normalized spectra are displayed in the right panels. Different colors are used for spectra acquired at different times. Some of them show obvious emissions in several lines of the Ca ii H & K, Hδ, Hγ, Hβ, Hα, and Ca ii IRT lines.
Fig. 3 Examples of spectra observed by LAMOST (left) and their EWs light curves (right) in the Hα, Hβ, Hγ, Hδ, Ca ii H & K and Ca ii IRT lines.

A variation of line intensity is clearly displayed by Figure 3. The EW variation might be due to the variation of chromospheric activity or accretion over the star surfaces. From the values of EWs, there is variation on a long-term scale of a year shown by FO Tau and FR Tau. These variations might be caused by accretion over the star surfaces. Indeed FO Tau and FR Tau (Andrews & Williams 2005 ; Wichmann et al. 1996 ; Furlan et al. 2011 ; Luhman et al. 2010 , 2017 ) possess a strong infrared excess which classifies them as classical T Tau stars with thick and dense circum-stellar disks and likely strong mass accretion onto the central star from the disk. An even more interesting point to note is that there is simultaneous radio and chromospheric activity for several objects. The observed radio emission exhibited variability on timescales of minutes to days in the 8−12 GHz radio light curves for dwarf nova type cataclysmic variables stars U Gem (Coppejans et al. 2016 ). The T Tau S source displays variable radio emission (Johnston et al. 2004 ). Several objects show both radio flux and chromospheric activity variability. We plotted the objects with radio and chromospheric variability in Figure 4, where the LAMOST observed spectra are on the left side and the continuum normalized spectra on the right.

Fig. 4 Observed LAMOST spectra of radio stars with variation in both radio flux and chromospheric activity.
4.2 Chromospheric Activity Properties in the Ca ii IRT Lines

The Ca ii IRT lines are very important chromospheric indicators, which are formed in the low chromosphere (e.g., Montes et al. 2000 ; Zhang et al. 2015 ). In our sample there are 14 radio stars with Ca ii IRT emission lines in Table 3.

No. Name Sp Source Named CaII8498 CaII8542 CaII8662 EW8542/EW8498
(1) (2) (3) (4) (5) (6) (7) (8) (9)
6 J041417.01+281057.5 K5 Wendker 1995 CW Tau 7.120 ± 0.18 12.808 ± 0.102 10.479 ± 0.099 1.799 ± 0.007
6 J041417.01+281057.5 K5 Wendker 1995 CW Tau 4.713 ± 0.007 6.829 ± 0.248 5.819 ± 0.169 1.449 ± 0.052
6 J041417.01+281057.5 K5 Wendker 1995 CW Tau 8.691 ± 0.091 13.395 ± 0.635 9.976 ± 0.094 1.541 ± 0.069
6 J041417.01+281057.5 K5 Wendker 1995 CW Tau 4.270 ± 0.01 7.222 ± 0.218 5.837 ± 1.899 1.691 ± 0.050
9 J041449.29+281230.2 M2 Wendker 1995 FO Tau 2.913 ± 0.097 3.595 ± 0.035 2.904 ± 0.025 1.234 ± 0.010
9 J041449.28+281230.5 M4 Wendker 1995 FO Tau 2.415 ± 0.325 2.542 ± 0.079 3.050 ± 0.143 1.053 ± 0.089
9 J041449.29+281230.2 M5 Wendker 1995 FO Tau 1.160 ± 0.11 1.564 ± 0.076 1.239 ± 0.015 1.348 ± 0.013
9 J041449.28+281230.5 M4 Wendker 1995 FO Tau 3.160 ± 0.09 4.118 ± 0.12 3.006 ± 0.011 1.303 ± 0.021
9 J041449.29+281230.2 M4 Wendker 1995 FO Tau 3.478 ± 0.078 4.207 ± 0.149 3.593 ± 0.07 1.210 ± 0.028
9 J041449.28+281230.5 M4 Wendker 1995 FO Tau 3.142 ± 0.078 3.641 ± 0.066 2.855 ± 0.034 1.159 ± 0.003
9 J041449.28+281230.5 M3 Wendker 1995 FO Tau 3.474 ± 0.054 4.039 ± 0.029 3.399 ± 0.056 1.163 ± 0.003
9 J041449.29+281230.2 M4 Wendker 1995 FO Tau 2.366 ± 0.096 3.268 ± 0.03 1.854 ± 0.176 1.381 ± 0.009
13 J041831.12+281628.4 M3 Wendker 1995 DD Tau ± ± ± − ± −
13 J041831.12+281628.4 M2 Wendker 1995 DD Tau 8.336 ± 0.484 10.480 ± 0.14 8.860 ± 0.209 1.257 ± 0.020
15 J041912.81+282933.0 M3 Wendker 1995 FQ Tau 7.345 ± 0.395 8.722 ± 0.392 8.378 ± 0.048 1.187 ± 0.015
15 J041912.80+282932.7 M4 Wendker 1995 FQ Tau ± ± ± − ± −
15 J041912.80+282932.7 M6 Wendker 1995 FQ Tau 4.852 ± 0.172 4.386 ± 0.087 3.778 ± 0.062 0.904 ± 0.025
19 J042155.63+275506.0 M0 Wendker 1995 DE Tau 6.827 ± 0.363 9.357 ± 0.196 8.080 ± 0.105 1.371 ± 0.001
23 J042923.73+243300.2 F4 Wendker 1995 GV Tau 19.447 ± 0.783 16.384 ± 0.616 16.227 ± 0.343 0.842 ± 0.025
23 J042923.73+243300.2 K4 Wendker 1995 GV Tau 17.890 ± 0.13 15.515 ± 0.435 16.198 ± 0.238 0.867 ± 0.015
31 J043138.47+181357.9 M0 Wendker 1995 HL Tau 25.185 ± 0.325 26.262 ± 1.192 17.327 ± 0.127 1.043 ± 0.035
34 J043215.40+242859.7 M0 Wendker 1995 V 806 Tau 1.909 ± 0.055 1.525 ± 0.027 1.534 ± 0.108 0.799 ± 0.031
34 J043215.40+242859.7 M0 Wendker 1995 V 806 Tau 3.053 ± 0.047 2.357 ± 0.059 2.456 ± 0.023 0.772 ± 0.007
41 J043828.58+261049.2 CV Wendker 1995 DO Tau 356.181 ± 14.281 438.184 ± 261.616 ± 1.230 ± 0.708
43 J043920.92+254501.8 M2 Wendker 1995 GN Tau 10.959 ± 0.731 13.390 ± 0.63 12.523 ± 0.017 1.222 ± 0.013
43 J043920.90+254502.1 M1 Wendker 1995 GN Tau 7.996 ± 0.671 3.078 ± 0.138 3.232 ± 0.365 0.385 ± 0.549
44 J043917.42+224753.2 K7 Wendker 1995 VY Tau 2.984 ± 0.065 2.523 ± 0.05 2.218 ± 0.076 0.846 ± 0.014
44 J043917.42+224753.2 K7 Wendker 1995 VY Tau 3.594 ± 0.035 2.895 ± 0.031 2.154 ± 0.731 0.806 ± 0.006
47 J044237.69+251537.0 K7 Wendker 1995 DP Tau 10.873 ± 0.527 9.428 ± 0.534 8.552 ± 0.198 0.867 ± 0.015
49 J045209.70+303745.1 CV Wendker 1995 Haro 6-39 12.240± 0.43 11.484 ± 0.306 7.981 ± 0.404 0.938 ± 0.015
52 J061250.35-061311.4 CV Wendker 1995 HH 1-2 50.221± 2.279 61.926 ± 1.316 41.578 ± 1.872 1.233 ± 0.004

Note: No value with ± means there is no data point.

Table 3 EWs of CaII IRT Lines for 14 Radio Stars

We list their relevant parameters: the number (Col. (1)), LAMOST name (Col. (2)), spectral type (Col. (3)), the reference (Col. (4)), other name (Col. (5)), EWs of three Ca ii IRT lines (Col. (6) to Col. (8)), and the EW8542/EW8498 ratio (Col. (9)) in Table 3.

The data indicate that these 14 radio stars have strong chromospheric activity emission. There have been many studies carried out on chromospheric activity in the Ca ii IRT lines, Ca ii H&K lines, and the Hα, Hβ and other hydrogen lines using the subtraction technique of non-active templates (e.g., Montes et al. 2000 ; Busà et al. 2007 ; Frasca et al. 2011 , 2016 ; Pi et al. 2016 ). We believe more active objects can be detected if the subtraction technique is applied, and many active objects are missing without considering the spectral subtraction. The stars with pure emission in the Ca ii IRT lines have a low effective temperature and, as a consequence, a low photospheric flux, which makes the line show emission. Eventually, they can be strong accretors. We hope to find more objects with strong emission in the Ca ii IRT lines and determine their statistical properties in the future. The value of the ratio EW8542/EW8498 is also an indicator of the optical thickness of the emitting plasma, which is helpful for distinguishing between stellar plages and prominences (see, e.g., Herbig & Soderblom 1980 ; Landman 1980 ). Our values of EW8542/EW8498 for the 14 radio stars fall mostly within the range 0.4–1.8 and are listed in the Col. (9) of Table 3. These small values mean that the chromospheric activity comes from optically thick emissions in probable stellar plage regions. These low ratios were also associated with several radio stars (e.g., Arévalo et al. 1999 ; Zhang et al. 2016 ).

4.3 Relationship between Chromospheric Activity and Radio Flux

Magnetic fields in the stellar interior produce chromospheric plages and coronal radio emissions. For different types of active stars, such as RS CVn systems (e.g., Fraquelli 1978 ; Su et al. 1994 ) and dMe stars (e.g., Gudel et al. 1989 , Osten et al. 2005 ), flare events have been detected using a combination of radio continuum, optical photometry, and optical, ultraviolet and X-ray spectroscopic observations. Radio outbursts may be preceded by Ha enhancement (e.g., Fraquelli 1978 ). There might be a correlation between strong chromospheric plages and strong flares in radio wavelengths. To examine the relationship between chromospheric activity and radio flux, we plot the chromospheric EWs of the Hα line, and the radio flux at 21 cm of radio stars from the FIRST survey (right), as well as the chromospheric EWs in the Hα line, and the radio flux at 6 cm of radio stars in Figure 5 (left) (e.g., Wendker 1995 ). Different symbols represent the results of different spectral types. As can be seen from Figure 5, there is no obvious trend in the chromospheric emission and radio coronal flux. The most likely reason for this behavior is that the observations at optical and radio wavelengths were not simultaneous and the sources are strongly variable. The radio flux, for instance, can go from a few mJy in a quiescent stage to several hundred mJy during flares, with timescales for bursts ranging from a few hours to a few days (e.g., Umana et al. 1995 ).

Fig. 5 Relationship between observed EWs of chromospheric activity indicators in the Hα line and radio flux of radio stars.

5 Summary

We analyzed 783 good stellar spectra for 610 radio stars by mining the LAMOST spectroscopic survey DR2. Using the EWs of the Hα line, we detected 89 objects with emission above the Hα continuum in 147 spectra. There are 36 objects with repeated observations, 28 of which show chromospheric activity variability. Furthermore, we found 14 radio stars showing emissions in the Ca ii IRT lines. This is the first catalog of radio stars observed in the LAMOST spectral survey. In the future, LAMOST will observe more radio stars, and we will update this catalog of radio stars. These new data will help us to address the study of activity variation at optical and radio wavelengths and their relation on better statistical grounds.


References

Andrews S. M. Williams J. P. 2005 ApJ 631 1134 (FO Tau)
Arévalo M. J. Lázaro C. 1999 AJ 118 1015
Berdyugina S. V. 2005 Living Reviews in Solar Physics 2 8
Berger E. 2002 ApJ 572 503
Berger E. 2006 ApJ 648 629
Berger E. Basri G. Gizis J. E. et al. 2008 ApJ 676 1307
Busà I. Aznar Cuadrado R. Terranegra L. Andretta V. Gomez M. T. 2007 A&A 466 1089
Butler C. J. Erkan N. Budding E. et al. 2015 MNRAS 446 4205
Carkner L. Mamajek E. Feigelson E. et al. 1997 ApJ 490 735
Coppejans D. L. Körding E. G. Miller-Jones J. C. A. et al. 2016 MNRAS 463 2229
Cui X.-Q. Zhao Y.-H. Chu Y.-Q. et al. 2012 RAA (Research in Astronomy and Astrophysics) 12 1197
Fernandez M. Ortiz E. Eiroa C. Miranda L. F. 1995 A&AS 114 439
Flesch E. Hardcastle M. J. 2004 A&A 427 387
Flesch E. 2010 PASA 27 283
Flesch E. W. 2016 PASA 33 e052
Fraquelli D. A. 1978 AJ 83 1535
Frasca A. Fröhlich H.-E. Bonanno A. et al. 2011 A&A 532 A81
Frasca A. Molenda-Żakowicz J. De Cat P. et al. 2016 A&A 594 A39
Furlan E. Luhman K. L. Espaillat C. et al. 2011 ApJS 195 3
Güudel M. 2002 ARA&A 40 217
Gudel M. Benz A. O. Bastian T. S. et al. 1989 A&A 220 L5
Gunn J. E. Carr M. Rockosi C. et al. 1998 AJ 116 3040
Hall J. C. 2008 Living Reviews in Solar Physics 5 2
Hawley S. L. Covey K. R. Knapp G. R. et al. 2002 AJ 123 3409
Helfand D. J. Schnee S. Becker R. H. White R. L. McMahon R. G. 1999 AJ 117 1568
Helfand D. J. White R. L. Becker R. H. 2015 ApJ 801 26
Herbig G. H. Soderblom D. R. 1980 ApJ 242 628
Hilton E. J. West A. A. Hawley S. L. Kowalski A. F. 2010 AJ 140 1402
Hughes V. A. 1988 ApJ 333 788
Ivezić Ž. Menou K. Knapp G. R. et al. 2002 AJ 124 2364
Johnston K. J. Fey A. L. Gaume R. A. et al. 2004 ApJ 604 L65
Karoff C. Knudsen M. F. De Cat P. et al. 2016 Nature Communications 7 11058
Kenyon S. J. Hartmann L. 1995 ApJS 101 117
Kimball A. E. Knapp G. R. Ivezić Ž. et al. 2009 ApJ 701 535
Landman D. A. 1980 ApJ 237 988
Luhman K. L. Allen P. R. Espaillat C. Hartmann L. Calvet N. 2010 ApJS 186 111
Luhman K. L. Mamajek E. E. Shukla S. J. Loutrel N. P. 2017 AJ 153 46
Luo A.-L. Zhao Y.-H. Zhao G. et al. 2015 RAA (Research in Astronomy and Astrophysics) 15 1095
McLean M. Berger E. Reiners A. 2012 ApJ 746 23
McMahon R. G. White R. L. Helfand D. J. Becker R. H. 2002 ApJS 143 1
Mohanty S. Jayawardhana R. Basri G. 2005 ApJ 626 498
Montes D. Fernández-Figueroa M. J. De Castro E. et al. 2000 A&AS 146 103
Nguyen D. C. Brandeker A. van Kerkwijk M. H. Jayawardhana R. 2012 ApJ 745 119
Osten R. A. Hawley S. L. Allred J. C. Johns-Krull C. M. Roark C. 2005 ApJ 621 398
Osten R. A. Hawley S. L. Allred J. et al. 2006 ApJ 647 1349
Paredes J. M. 2005 EAS Publications Series 15 EAS Publications Series Gurvits L. I. Frey S. Rawlings S. 187
Pi Q. F. Zhang L. Y. Chang L. et al. 2016 RAA (Research in Astronomy and Astrophysics) 16 153
Skrutskie M. F. Cutri R.M. Stiening R. et al. 2006 AJ 131 1163
Su B.-M. Mutel R. L. Li Y.-S. Zhang F.-J. 1994 Chinese Astronomy and Astrophysics 18 11
Umana G. Trigilio C. Tumino M. Catalano S. Rodono M. 1995 A&A 298 143
Wang S.-G. Su D.-Q. Chu Y.-Q. Cui X. Wang Y.-N. 1996 Appl. Opt. 35 5155
Wendker H. J. 1987 A&AS 69 87
Wendker H. J. 1995 A&AS 109 177
Wendker H. J. 2015 VizieR Online Data Catalog 8099
West A. A. Hawley S. L. Walkowicz L. M. et al. 2004 AJ 128 426
West A. A. Morgan D. P. Bochanski J. J. et al. 2011 AJ 141 97
White S. M. Jackson P. D. Kundu M. R. 1989 ApJS 71 895
Wichmann R. Krautter J. Schmitt J. H. M. M. et al. 1996 A&A 312 439
Wu Y. Luo A.-L. Li H.-N. et al. 2011 RAA (Research in Astronomy and Astrophysics) 11 924
Wu Y. Luo A. Du B. Guo Y. 2014 IAU Symposium 298 Setting the Scene for Gaia and LAMOST Feltzing S. Zhao G. Walton N. A. Whitelock P. 445
Yi Z. Luo A. Song Y. et al. 2014 AJ 147 33
York D. G. Adelman J. Anderson J. E. Jr. et al. 2000 AJ 120 1579
Zhang L.-Y. Gu S.-H. 2008 A&A 487 709
Zhang L. Pi Q. Zhu Z. Z. 2015 RAA(Research in Astronomy and Astrophysics) 15 252
Zhang L. Pi Q. Han X. L. et al. 2016 New Astron. 44 66
Zhao G. Zhao Y.-H. Chu Y.-Q. Jing Y.-P. Deng L.-C. 2012 RAA(Research in Astronomy and Astrophysics) 12 723
Cite this article: Zhang Li-Yun, Yue Qiang, Lu Hong-Peng, Han Xian-Ming L., Zhang Yong, Shi Jian-Rong, Wang Yue-Fei, Hou Yong-Hui, Zi-Huang Cao. Radio stars observed in the LAMOST spectral survey. Cancer Biol Med. 2017; 10:105.

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