GMRT Detections of Low-Mass Young Stars at 323 and 608 Mhz
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MNRAS 459, 1248–1258 (2016) doi:10.1093/mnras/stw708 Advance Access publication 2016 March 28 GMRT detections of low-mass young stars at 323 and 608 MHz Rachael E. Ainsworth,1‹ Anna M. M. Scaife,2 David A. Green,3 Colm P. Coughlan1 and Tom P. Ray1 1Dublin Institute for Advanced Studies, School of Cosmic Physics, 31 Fitzwilliam Place, Dublin D02 XF86, Ireland 2Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, The University of Manchester, Oxford Road, Manchester M13 9PL, UK 3Astrophysics Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK Accepted 2016 March 22. Received 2016 March 21; in original form 2016 February 10 ABSTRACT We present the results of a pathfinder project conducted with the Giant Metrewave Radio Telescope (GMRT) to investigate protostellar systems at low radio frequencies. The goal of these investigations is to locate the break in the free–free spectrum where the optical depth equals unity in order to constrain physical parameters of these systems, such as the mass of the ionized gas surrounding these young stars. We detect all three target sources, L1551 IRS 5 (Class I), T Tau and DG Tau (Class II), at frequencies 323 and 608 MHz (wavelengths 90 and 50 cm, respectively). These are the first detections of low-mass young stellar objects at such low frequencies. We combine these new GMRT data with archival information to construct the spectral energy distributions for each system and find a continuation of the optically thin free–free spectra extrapolated from higher radio frequencies to 323 MHz for each target. We use these results to place limits on the masses of the ionized gas and average electron densities associated with these young systems on scales of ∼1000 au. Future observations with higher angular resolution at lower frequencies are required to constrain these physical parameters further. Key words: radiation mechanisms: thermal – stars: individual: L1551 IRS 5 – stars: individ- ual: T Tauri – stars: individual: DG Tauri – stars: pre-main-sequence – radio continuum: stars. outflows (e.g. Anglada, Rodr´ıguez & Carrasco-Gonzalez 2015). 1 INTRODUCTION The free–free spectrum is characterized by a flat or positive power- α The youngest protostars (Classes 0 and I) are deeply embedded law spectral index α (where the flux density Sν ∝ ν at frequency within embryonic envelopes and discs from which they accrete. ν) and ranges between −0.1 for optically thin emission and +2 They drive powerful bipolar outflows which likely remove angular for optically thick emission. A value of α ≈ 0.6 is the canonical momentum from the system and allow accretion to proceed. Class II value for a constant velocity, isothermal, spherical (or wide-angle) protostars, also known as classical T Tauri stars, drive less powerful wind (e.g. Panagia & Felli 1975) and values less than this (e.g. α ≈ outflows as most of their mass has been accreted, but they are still 0.25) are expected for more collimated flows (Reynolds 1986). The surrounded by an optically thick disc (Andre,´ Ward-Thompson & HH objects typically exhibit optically thin free–free emission due Barsony 2000). These outflows exhibit shocks and knots of many to shock ionization (e.g. Curiel, Canto & Rodr´ıguez 1987;Curiel irregular morphologies along their lengths which can stretch to 1995; Rodr´ıguez et al. 2000). several parsecs and can terminate in bright bow shocks. The optical The study of YSOs at radio wavelengths has been predominantly manifestations of these shocks are the so-called Herbig–Haro (HH; constrained to frequencies >1 GHz (λ<30 cm). This is due to the see e.g. Frank et al. 2014) objects, and they have been detected past sensitivity limitations of radio telescopes, the radio weakness in many bands of the electromagnetic spectrum from X-ray (e.g. of YSOs (flux densities 1 mJy at centimetre wavelengths) and Pravdo et al. 2001) to radio wavelengths (e.g. Rodr´ıguez et al. the fact that their flux densities typically rise with frequency. How- 2000). ever, recent surveys of nearby star-forming regions with the Karl The radio emission from young stellar objects (YSOs) is typi- G. Jansky Very Large Array (VLA) find that at least half of the cally detected as free–free radiation from the collimated, ionized detected pre-main-sequence objects are non-thermal sources with active coronae characterized by high levels of variability, negative spectral indices, and in some cases significant circular polarization E-mail: [email protected] (e.g. Dzib et al. 2013, 2015). This is consistent with more evolved C 2016 The Authors Downloaded from https://academic.oup.com/mnras/article-abstract/459/2/1248/2595082Published by Oxford University Press on behalf of the Royal Astronomical Society by University of Cambridge user on 15 November 2017 GMRT detections of YSOs 1249 (Class III) objects which have already shed most of their surround- very low power jet against the ambient medium, was presented in ing material and are detectable via the gyrosynchrotron emission Ainsworth et al. (2014). from their coronae. In this paper, we present the lowest frequency detections of YSOs Yet there are a small number of cases where non-thermal emission made to date. In Section 2, we present details of the observations and (α −0.1) is seen from Classes 0 to II YSOs. Ray et al. (1997) data reduction. In Section 3, we present radio images made at 323 detected circularly polarized gyrosynchrotron emission from the and 608 MHz (90 and 50 cm, respectively) of the three target YSOs young embedded object T Tauri S, indicating that strong magnetic with the GMRT, measure their associated flux densities and calcu- fields and at least mildly relativistic particles are present in the late their spectral indices. In Section 4, we discuss the detections and extended outflow of this low-mass YSO. Furthermore, Carrasco- non-detections, and model the spectral energy distributions (SEDs) Gonzalez´ et al. (2010) detected linearly polarized emission from for each target source. We find that these new GMRT detections the high-mass protostellar jet, HH 80-81, which allowed for the show a continuation of the free–free spectrum extrapolated from direct measurement of the magnetic field direction and estimates higher frequencies, and that the frequency at which the emission of its strength in this outflow. These results are remarkable, as the transitions between optically thin and thick behaviour has not yet presence of relativistic particles was previously unexpected from been reached. We therefore place limits on the mass of ionized gas, these outflows which have typical shock velocities of order ∼30– the average electron density and the emission measure, assuming 300 km s−1. More importantly, they shed light on the large-scale that the turnover frequency is less than 323 MHz. We make our magnetic field which has remained an elusive characteristic of these concluding remarks in Section 5. systems. Finally, there are a handful of low-mass Classes 0–II YSOs with outflow components which exhibit spectral indices indicative 2 OBSERVATIONS AND DATA REDUCTION of non-thermal emission (e.g. Curiel et al. 1993;Girartetal.2002), however linearly polarized emission has yet to be detected in the The observations were made using the GMRT (see e.g. Ananthakr- outflow from a low-mass YSO. ishnan 2005), which consists of thirty 45 m dishes. 14 of the 30 We conducted a pathfinder project with the Giant Metrewave dishes are arranged in a compact, quasi-randomly distributed cen- Radio Telescope (GMRT) to search for YSOs at very low ra- tral array within a square kilometre. The remaining 16 dishes are dio frequencies (<1 GHz). We observed a sample of three well- spread out along three arms of an approximately ‘Y’-shaped con- studied low-mass YSOs with known radio emission previously figuration, with a longest interferometric baseline of about 25 km. measured at frequencies >1 GHz: L1551 IRS 5, T Tau and DG Observations of the young stars L1551 IRS 5, T Tau and DG Tau. All three reside in the Taurus Molecular Cloud at distances Tau were made at 325 and 610 MHz between 2012 December 6 of approximately 130–140 pc (Kenyon, Dobrzycka & Hartmann and 14 (average epoch 2012.95). At 325 MHz, observations were 1994;Torresetal.2012). The GMRT has a spatial resolution taken for 7 h per night over the course of three nights for a total of approximately 10 and 5 arcsec at 325 and 610 MHz, respec- of 21 h and observations at 610 MHz were taken for 10 h in a tively. We are therefore exploring these systems on large scales single run. See Table 1 for the number of hours on-source for each of ∼700–1400 au. target. A total bandwidth of 32 MHz was observed, which was split The importance of investigating the radio emission from young into 256 spectral channels. The sample integration time was 16.9 s. stars at low radio frequencies is twofold. First, observations of The primary beam of the GMRT has a full width at half-maximum the low-frequency turnover in the free–free spectrum (at frequency (FWHM) of approximately 85 arcmin at 325 MHz and 44 arcmin ν0) can constrain physical properties of the ionized plasma asso- at 610 MHz. The observational details can be found in Table 1. ciated with these systems. Where the free–free emitting plasma is The flux density scale was set through observations of 3C48 at reasonably uniform in density, there is a characteristic radio spec- the beginning and end of each observing run. For the cases where trum which has two components, optically thick behaviour at lower 3C48 set before the end of the run, 3C147 or 3C286 were observed.