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AMI Radio Continuum Observations of Young Stellar Objects with Known Outflows

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AMI Radio Continuum Observations of Young Stellar Objects with Known Outflows Mon. Not. R. Astron. Soc. 000, 1–?? (2012) Printed 11 August 2018 (MN LATEX style file v2.2) AMI radio continuum observations of young stellar objects with known outflows⋆ AMI Consortium: Rachael E. Ainsworth1†, Anna M. M. Scaife2, Tom P. Ray1, Jane V. Buckle3,4, Matthew Davies3, Thomas M. O. Franzen5, Keith J. B. Grainge3,4, Michael P. Hobson3, Natasha Hurley-Walker6, Anthony N. Lasenby3,4, Malak Olamaie3, Yvette C. Perrott3, Guy G. Pooley3, John S. Richer3,4, Carmen Rodr´ıguez-Gonzalvez´ 7, Richard D. E. Saunders3,4, Michel P. Schammel3, Paul F. Scott3, Timothy Shimwell5, David Titterington3, Elizabeth Waldram3. 1 Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland 2 School of Physics & Astronomy, University of Southampton, Highfield, Southampton, SO17 1BJ 3 Astrophysics Group, Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE 4 Kavli Institute for Cosmology, Cambridge, Madingley Road, Cambridge, CB3 0HA 5 CSIRO Astronomy & Space Science, Australia Telescope National Facility, PO Box 76, Epping, NSW 1710, Australia 6 International Centre for Radio Astronomy Research, Curtin Institute of Radio Astronomy, 1 Turner Avenue, Technology Park, Bentley, WA 6845, Australia 7 Spitzer Science Center, MS 220-6, California Institute of Technology, Pasadena, CA 91125 Accepted —; received —; in original form 11 August 2018 ABSTRACT We present 16GHz (1.9cm) deep radio continuum observations made with the Arcminute Microkelvin Imager (AMI) of a sample of low-mass young stars driving jets. We combine these new data with archival information from an extensive literature search to examine spec- tral energy distributions (SEDs) for each source and calculate both the radio and sub-mm spectral indices in two different scenarios: (1) fixing the dust temperature (Td) according to evolutionary class; (2) allowing Td to vary. We use the results of this analysis to place con- straints on the physical mechanisms responsible for the radio emission. From AMI data alone, as well as from model fitting to the full SED in both scenarios, we find that 80 per cent of the objects in this sample have spectral indices consistent with free-free emission. We find an average spectral index in both Td scenarios, consistent with free-free emission. We examine arXiv:1203.3381v1 [astro-ph.SR] 15 Mar 2012 correlations of the radio luminosity with bolometric luminosity, envelope mass, and outflow force and find that these data are consistent with the strong correlation with envelope mass seen in lower luminosity samples. We examine the errors associated with determining the ra- dio luminosity and find that the dominant source of error is the uncertainty on the opacity index, β. We examine the SEDs for variability in these young objects, and find evidence for possible radio flare events in the histories of L1551 IRS 5 and Serpens SMM 1. Key words: radiation mechanisms: general — ISM: general — ISM: clouds — stars: forma- tion 1 INTRODUCTION protostellar evolution (Andr´eet al. 1993) in which the protostar has yet to accrete most of its mass from the surrounding envelope, Young stellar objects (YSOs) are divided into a number of evolu- and therefore the central object is less massive than the envelope tionary classes. Class 0 protostars represent the youngest phase of (Menv > M ). The subsequent Class I phase (Lada 1987) occurs when a majority∗ of the remainder of the envelope has accreted onto the protostar or its circumstellar disc (Menv < M ). The Class II ⋆ We request that any reference to this paper cites “AMI Consortium: phase (Lada 1987), also known as the Classical T∗ Tauri stage, oc- Ainsworth et al. 2012”. curs when infall is almost complete and the central object is em- † email: Email [email protected] c 2012 RAS 2 Ainsworth et al. bedded in an optically thick disc and enters the pre-main-sequence with the other global properties of these systems, such as bolomet- (PMS) stage of evolution. Embedded YSOs are often found to have ric luminosity, envelope mass, and outflow force (AMI Consortium: partially ionised and collimated outflows/jets driven by the accre- Scaife et al. 2011a,b, 2012b). If such free-free emission is to be rec- tion from their envelopes, and are detectable from molecular gas onciled with ionization mechanisms arising from the impact of the tracers such as CO and by the shocks along the length of the out- molecular outflow or protostellar jet/wind on its surroundings then flows. The outflows driven by Class I protostars are less powerful it is vital to measure it in its entirety to determine quantitatively its than those of Class 0 due to a decline in the accretion rate (Bon- physical correspondence to other global properties of the protostel- temps et al. 1996). lar system, in much the same way that radio emission from massive The target sample in this work is comprised of Class 0 and young stellar objects can be used to infer the total ionizing flux of Class I protostars. Since the mass ratio Menv > M is not directly the central object and hence its spectral type. Such indirect mea- observable, Andr´eet al. (1993, 2000) define the following∗ observa- sures of protostellar activity in the early embedded phase of YSO tional properties required to identify an object as Class 0. The first lifetimes may provide a window into the evolution of these objects, is the detection of a compact centimetre radio continuum source, which is otherwise obscured. a collimated CO outflow, or an internal heating source as indirect In this paper we investigate the properties of the integrated ra- evidence for a central YSO. These criteria distinguish the central dio emission from this sample of low-mass YSOs. In general, these object from a starless prestellar core. The second requirement is new data confirm trends found in previous works and improve upon centrally peaked but extended submillimeter continuum emission the statistics. We identify outliers which may provide useful tar- tracing the presence of a spheroidal circumstellar dust envelope (as gets for further study at high spatial resolution. We combine our opposed to just a disc). The third is a high ratio of submillimeter to results for the flux densities with those found in an extensive liter- bolometric luminosity, corresponding to an envelope mass greater ature search to calculate the spectral indices at radio wavelengths, than the stellar mass. Class 0 spectral energy distributions (SEDs) and which are overwhelmingly consistent with free-free emission are typically well characterized by single temperature blackbodies as the mechanism for the radio emission. with 15 6 Td 6 30 K and where Td is the dust temperature. The sec- The organization of this paper is as follows. In §2 we describe ond and third criteria differentiate Class 0 sources from the more the sample of targets to be observed. In §3 we describe the AMI evolved Class I and II objects. For further distinction, bolometric telescope, the observations, and the data reduction process. In §4 temperature is also used to characterise evolutionary class, where we comment on the results of the observations and §5 contains de- Tbol < 70 K for Class 0 objects and 70 < Tbol < 650 K for Class I tailed notes on the individual fields. In §6 we discuss the detections (Chen et al. 1995). and non-detections, the expected contamination from extragalactic The radio counterpart at centimetre wavelengths for these sources, the spectral energy distributions, the radio spectral indices, low-mass young stars is detected almost certainly via thermal the derivation of physical parameters, correlations between the ra- bremsstrahlung radiation. This “free-free”emission is typically ob- dio luminosity and other global properties, and finally, evidence for served to be compact and elongated along the outflow axis, lead- variability. We make concluding remarks in §7. ing to the designation of these sources as “‘thermal radio jets”’(e.g. Rodr´ıguez 1995). Free-free emission at these wavelengths typically has a flat or positive power-law spectral index α, where the flux α 2 THESAMPLE density Sν ∝ ν at frequency ν. It has been shown that α = 0.6 for a standard conical jet, and α < 0.6 for an unresolved, partially This study targets eleven YSOs driving known outflows. The coor- opaque flow where the cross-sectional area grows more slowly than dinates of each of these objects are listed in Columns [3] and [4] length (Reynolds 1986; Anglada et al. 1998). However, in a num- of Table 1 along with their identifier. We follow the classification ber of cases non-thermal emission is also seen (e.g. Ray et al. 1997; scheme described in Hatchell et al. (2007b) for Class 0 or Class I Carkner et al. 1997; Feigelson et al. 1998; Carrasco-Gonz´alez et al. protostars, based on three evolutionary indicators, which for a 2010a). There are some objects that have yielded spectral indices Class I are Tbol > 70K, Lbol/Lsmm > 3000, and F3.6/F850 > 0.003, too negative to be explained by free-free emission alone, such as where F3.6 is the flux at 3.6 cm and and F850 is the flux at 850 µm. the Serpens MMS 1 triple radio source (Rodr´ıguez et al. 1989b; There are eight Class 0 and three Class I protostars in the target Curiel et al. 1993; AMI Consortium: Scaife et al. 2012b), suggest- sample, listed in Table 1. A number of additional sources were de- ing that non-thermal processes must contribute. The radio emission tected within the AMI fields. We identify these additional sources can also be attributed, at least at very short wavelengths, to the discs assuming a generous threshold of 10 arcsec of a known protostel- around these young stars (Rodr´ıguez et al.
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