Radio stars observed in the LAMOST spectral survey

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.

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.

We calculated the equivalent widths (EWs) of the spectral lines using the usual formula

$${\rm EW}=\displaystyle \int_{\rm line} \frac{F_{\lambda}-F_C}{F_C} d\lambda,$$

where F_λ 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 software¹. We plotted the LAMOST observed (left panels) and their normalized continuum spectra (right panels) in Figure 1.

Zoom In Zoom Out Reset image size

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.

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.

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.

Zoom In Zoom Out Reset image size

Zoom In Zoom Out Reset image size

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.

Zoom In Zoom Out Reset image size

4.2. Chromospheric Activity Properties in the Ca ii IRT Lines

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).

Zoom In Zoom Out Reset image size

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.

This work is supported by the Joint Research Fund in Astronomy (U1631236, U1431114, U1631109 and 11263001) under cooperative agreement between NSFC and Chinese Academy of Sciences (CAS). Our paper used JHK magnitudes from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. The Guo Shou Jing Telescope (LAMOST) is a National Major Scientific Project built by CAS. Funding for the project has been provided by the National Development and Reform Commission. LAMOST is operated and managed by NAOC, CAS.