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First Detection of Thermal Radio Emission from Solar Draft version November 14, 2013 Preprint typeset using LATEX style emulateapj v. 04/17/13 FIRST DETECTION OF THERMAL RADIO EMISSION FROM SOLAR-TYPE STARS WITH THE JANSKY VLA Jackie Villadsen, Gregg Hallinan, Stephen Bourke Astronomy Department, California Institute of Technology, Pasadena, CA 91125 Draft version November 14, 2013 ABSTRACT We present the first detections of thermal radio emission from the atmospheres of solar-type stars τ Cet, η Cas A, and 40 Eri A. These stars all resemble the Sun in age and level of magnetic activity, as indicated by X-ray luminosity and chromospheric emission in calcium H and K lines. We observed these stars with the Jansky VLA with sensitivities of a few µJy at combinations of 10.0, 15.0, and 34.5 GHz. τ Cet, η Cas A, and 40 Eri A are all detected at 34.5 GHz with signal-to-noise ratios of 7.6, 4.6, and 4.2, respectively. 15.0-GHz upper limits imply a rising spectral index greater than 1.0 for τ Cet and 1.7 for η Cas A, at the 99% confidence level. The measured 34.5-GHz fluxes correspond to stellar disk-averaged brightness temperatures of roughly 10,000 K, similar to the solar brightness temperature at the same frequency. We explain this emission as optically-thick thermal free-free emission from the chromosphere, with possible contributions from coronal gyroresonance emission above active regions and coronal free-free emission. 1. INTRODUCTION a radio luminosity roughly 10 times that of the quiet Efforts to measure radio emission from the Sun started Sun due to its larger surface area and somewhat higher as early as 1896 [Wilsing & Scheiner (1896)]. The first brightness temperature. detections occurred during the boom in radio technology In low-mass main-sequence stars, the only form of development during World War II, when military radio quiescent radio emission as of yet detected is gyrosyn- engineers James Stanley Hey (1946) in Britain, George chrotron emission from a persistent non-thermal corona, Clark Southworth (1945) in the United States, and Bruce a feature which has no analog in the Sun. G¨udelet al. Slee in Australia (Orchiston (2005)) all independently (1994) reported the detection of 8.5-GHz emission from identified solar radio emission as a source of interference solar-type stars, with radio luminosities a few thousand in their radar signals. times that of the Sun. These stars also have X-ray lu- Seventy years later, solar radio observations have con- minosities much higher than the Sun, following an X- tributed significantly to a detailed (although far from ray-radio luminosity relation observed by G¨udel& Benz complete) understanding of the solar atmosphere. So- (1993) in quiescent emission from coronae of active stars lar flares produce transient radio emission from MHz to with spectral type F to M. Benz & G¨udel(1994) ob- GHz frequencies, including gyrosynchrotron storms and served that the X-ray-radio luminosity relation is also coherent bursts, which act as diagnostics of electron den- seen in solar flares, suggesting that non-thermal stellar sity and magnetic field strength in the solar corona (see radio \coronae" may consist of the emission from many Bastian et al. (1998) for a review). small flares, or at least are continuously heated by flares. The microwave emission from the quiet Sun consists Previous to the results reported here, the most sen- of thermal radiation from the chromosphere at ∼ 104 K, sitive microwave observations of solar-type stars with which is optically thick at microwave frequencies due to solar-like X-ray luminosities were performed with the free-free opacity. The quiet-Sun brightness temperature Very Large Array (VLA) by G¨udel (1992), placing a 3- sigma upper limit of 80 µJy on the 8.3-GHz flux of 40 spectrum probes atmospheric temperature and density 1 from the temperature minimum (far-IR) to the upper Eridani A. With the enhanced sensitivity of the Jansky chromosphere (cm wavelengths), as modeled in Louk- VLA, the observations described in this paper reached a itcheva et al. (2004). During periods of heightened solar 3-sigma detection limit of 6 to 12 µJy in a few hours of activity, low-frequency solar radiation (below 10-20 GHz) observation, enabling the first detections of microwave is enhanced by bright spots above active regions, where radio emission from the thermal atmospheres of solar- the ∼ 106-K corona is optically thick to gyroresonant type stars. and/or free-free opacity. For this reason, the 10.7-cm so- In this paper we present the first detections of ther- mal radio emission from three nearby solar-type stars lar flux F10:7 is a traditional measure of solar activity, varying by a factor of 2 to 3 during the solar cycle. with solar-like levels of magnetic activity: τ Ceti, η Cas- Radio luminosities at the level of the quiet Sun have yet siopeiae A, and 40 Eridani A. In Section 2, we review to be detected in other cool main-sequence stars because basic stellar parameters and activity indicators of our of the low fluxes: a few tens of µJy or less for stars at a sample, describe the observing program, and discuss the few parsecs. Just beyond the main sequence, Drake et al. (1993) detected 8.3-GHz thermal chromospheric emission 1 In contrast, these stars are easily detected at sub-mm wave- lengths, where the optically-thick thermal emission from the low from Procyon, a slightly-evolved F5 subgiant, which has chromosphere is of order a few mJy, and which must be accounted for in sub-mm observations of debris disks, as for τ Cet in Greaves [email protected] et al. (2004). 2 Name HD Distance (pc) Spectral Type Mass (M ) Radius (R ) Teff (K) [Fe/H] Known Stellar Companions τ Cet 10700 3.65 a G8.5V b 0.783 c 0.790 d 5400 b -0.40 b none η Cas A 4614 5.95 a F9V e 0.972 f 1.039 f 6000 f -0.25 g K7V @ 70 AU (12") h 40 Eri A 26965 4.98 a K0.5V b 0.84 i 0.77 j 5100 b -0.28 b DA3 & M5Ve @ 400 AU (80") k Sun | 4.8e-6 G2V 1 1 5700 0 none aBased on stellar parallaxes from van Leeuwen (2007). bGray et al. (2006). cTeixeira et al. (2009). ddi Folco et al. (2007). eGray et al. (2001b). fBoyajian et al. (2012). gGray et al. (2001a). hMason et al. (2013). The distances of stellar companions are reported as the orbital semi-major axis. iHolmberg et al. (2007). jDemory et al. (2009). kHeintz (1974). TABLE 1 Basic stellar properties. probability of chance alignment of extragalactic sources. from the National Radio Astronomy Observatory's Very Section 3 presents the source detections at various fre- Large Array (VLA) at latitude 34.1◦ N: τ Cet, η Cas A, quencies, draws a comparison to the solar spectrum, and 40 Eri A. Tables 1 and 2 compare these stars' prop- and considers possible causes for a discrepancy in ra- erties, including a number of measures of stellar activity, dio and optical source coordinates. Section 4 discusses to those of the Sun. All three stars are a good match for the various possible radio emission mechanisms for the the Sun in age and activity level. detected sources: chromospheric disk emission, gyrores- onance emission above active regions, free-free emission 2.2. List of Observations from the corona and stellar wind, gyrosynchrotron emis- Table 3 summarizes the observations of all stars in the sion from a non-thermal corona, or flare emission. In sample. Each star in the sample was observed with some Section 5, we conclude with a review of the detected subset of X band (8.0 - 12.0 GHz), Ku band (12.0 - 18.0 sources and most likely emission mechanisms, and dis- GHz), and Ka band (tuned to 30.5 - 38.5 GHz). The cuss the potential for future observations with the VLA, VLA WIDAR correlator's 3-bit observing mode enabled ALMA, and the SKA. up to 8-GHz bandwidth. Observations were performed between March and September 2013, with the VLA in 2. OBSERVATIONS D array and C array as well as intermediate configura- 2.1. Sample tions. Observations alternated between a nearby phase Our sample consists of three of the nearest stars of calibrator and the target source with cycle times of 7.5 spectral type F9V through K0.5V that are observable minutes in Ka band and 17 minutes in X and Ku band. Typical sensitivities obtained with the full bandwidth in Name log L P (days) log R0 Age (Gyr) 10 X rot 10 HK one hour were 4, 5, and 9 µJy in X, Ku, and Ka bands, (erg/s) respectively. τ Cet 26.5 a 34 b -5.01 c 5.8 d η Cas A 27.4 a 14.96 e -4.93 c 2.9 d 2.3. Source Motion 40 Eri A 27.2 a 43 b -4.872 b 5.6 d The expected positions of the sources were determined Sun 27.35 f 26.1 g -4.906 d 4.6 h using Hipparcos coordinates and proper motions (van Leeuwen (2007)), with an additional correction for par- TABLE 2 allactic motion based on the distances in Table 1. In Measures of stellar magnetic activity. addition, the position of η Cas A was corrected for dis- placement due to orbital acceleration, which resulted in aStellar X-ray luminosities are from pointed observations by the a 0.3" displacement northwest of the position expected ROSAT High-Resolution Imager (HRI) as reported by Schmitt & Liefke (2004) in the NEXXUS catalog.
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