The Astrophysical Journal, 743:201 (14pp), 2011 December 20 doi:10.1088/0004-637X/743/2/201 C 2011. The American Astronomical Society. All rights reserved. Printed in the U.S.A. THE ENIGMATIC CORE L1451-mm: A FIRST HYDROSTATIC CORE? OR A HIDDEN VeLLO?��� Jaime E. Pineda1,7, Hector �� G. Arce2, Scott Schnee3, Alyssa A. Goodman1, Tyler Bourke1, Jonathan B. Foster1,8, Thomas Robitaille1, Joel Tanner2, Jens Kauffmann1,9, Mario Tafalla4, Paola Caselli5, and Guillem Anglada6 1 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA jaime.pineda@manchester.ac.uk 2 Department of Astronomy, Yale University, P.O. Box 208101, New Haven, CT 06520-8101, USA 3 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA 4 Observatorio Astronomico �� Nacional (IGN), Alfonso XII 3, E-28014 Madrid, Spain 5 School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK 6 Instituto de Astrof�� ��sica de Andaluc�� ��a, CSIC, Apartado 3004, E-18080 Granada, Spain Received 2011 March 29 accepted 2011 September 6 published 2011 December 6 ABSTRACT We present the detection of a dust continuum source at 3 mm (CARMA) and 1.3 mm (Submillimeter Array, SMA), and 12CO (2���1) emission (SMA) toward the L1451-mm dense core. These detections suggest a compact object and an outflow where no point source at mid-infrared wavelengths is detected using Spitzer. An upper limit for the dense core bolometric luminosity of 0.05 L is obtained. By modeling the broadband spectral energy distribution and the continuum interferometric visibilities simultaneously, we confirm that a central source of heating is needed to explain the observations. This modeling also shows that the data can be well fitted by a dense core with a young stellar object (YSO) and a disk, or by a dense core with a central first hydrostatic core (FHSC). Unfortunately, we are not able to decide between these two models, which produce similar fits. We also detect 12CO (2���1) emission with redshifted and blueshifted emission suggesting the presence of a slow and poorly collimated outflow, in opposition to what is usually found toward YSOs but in agreement with prediction from simulations of an FHSC. This presents the best candidate, so far, for an FHSC, an object that has been identified in simulations of collapsing dense cores. Whatever the true nature of the central object in L1451-mm, this core presents an excellent laboratory to study the earliest phases of low-mass star formation. Key words: ISM: clouds ��� ISM: individual objects (L1451, Perseus) ��� ISM: molecules ��� stars: formation ��� stars: low-mass Online-only material: color figures 1. INTRODUCTION Star formation takes place in the densest regions of molecular clouds, usually referred to as dense cores. The parental molec- ular clouds show highly supersonic velocity dispersions, while the dense cores show subsonic levels of turbulence (Goodman et al. 1998 Caselli et al. 2002). Recently, Pineda et al. (2010) showed that this transition in velocity dispersion is extremely sharp and it can be observed in NH3 (1,1) (see also J. E. Pineda et al. 2011, in preparation). Starless dense cores represent the initial conditions of star formation. Crapsi et al. (2005) identify a sample of starless ��� Based on observations carried out with the IRAM 30 m Telescope, the Submillimeter Array, and CARMA. IRAM is supported by INSU/CNRS (France), MPG (Germany), and IGN (Spain). The Submillimeter Array is a joint project between the Smithsonian Astrophysical Observatory and the Academia Sinica Institute of Astronomy and Astrophysics and is funded by the Smithsonian Institution and the Academia Sinica. Support for CARMA construction was derived from the states of California, Illinois, and Maryland, the James S. McDonnell Foundation, the Gordon and Betty Moore Foundation, the Kenneth T. and Eileen L. Norris Foundation, the University of Chicago, the Associates of the California Institute of Technology, and the National Science Foundation. Ongoing CARMA development and operations are supported by the National Science Foundation under a cooperative agreement and by the CARMA partner universities. 7 Current address: ESO, Karl Schwarzschild Str. 2, 85748 Garching bei Munchen, Germany and UK ALMA Regional Centre Node, Jodrell Bank Centre for Astrophysics, School of Physics and Astronomy, University of Manchester, Manchester M13 9PL, UK. 8 Current address: Institute for Astrophysical Research, 725 Commonwealth Avenue, Boston, MA 02215, USA. 9 Current address: NPP Fellow, Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, USA. cores which show a number of signs indicating that they may be ���evolved��� and thus close to forming a star. In the earliest phases of star formation a starless core undergoes a gravitational collapse. Increasing central densities will result in an increase in dust optical depth and thus cooling within the core will not be as efficient as in the earliest phases. This increases the gas temperature and generates more pressure. The first numerical simulation to study the formation of a protostar from an isothermal core (Larson 1969), revealed the formation of a central adiabatic core, defined as a ���first hydrostatic core��� (hereafter FHSC). This FHSC would then accrete more mass and undergo adiabatic contraction until H2 is dissociated, at which point it begins a second collapse until it forms a ���second hydrostatic core,��� which is the starting point for protostellar objects. A few FHSC candidates have been suggested in the past. Belloche et al. (2006) present single-dish observations of the Cha-MMS1 dense core which combined with detections at 24 and 70 ��m with Spitzer suggest the presence of an FHSC or an extremely young protostar (see also Belloche et al. 2011). Chen et al. (2010) present Submillimeter Array (SMA) observations of the continuum at 1.3 mm and 12CO (2���1) line in the L1448 region located in the Perseus cloud where no Spitzer (Infrared Array Camera, IRAC, or Multiband Imaging Photometer for Spitzer (MIPS Rieke et al. 2004)) source is detected. They detect a weak continuum source and a well- collimated high-velocity outflow is observed in 12CO (2���1). Chen et al. (2010) analyze different scenarios to explain the observations and conclude that an FHSC provide the best case however, no actual modeling of the interferometric observations 1

The Astrophysical Journal, 743:201 (14pp), 2011 December 20 Pineda et al. is presented. Recently, Enoch et al. (2010) present CARMA 3 mm continuum and deep Spitzer 70 ��m observations of another FHSC candidate (Per-Bolo 58) in the NGC1333 region also located in the Perseus cloud. In these observations, they detect a weak source in the 3 mm continuum and 70 ��m. Enoch et al. (2010) simultaneously modeled the broadband spectral energy distribution (SED) and the visibilities, allowing them to conclude that the best explanation for the central source is an FHSC. Dunham et al. (2011) present SMA 1.3 mm observations which reveal a collimated slow molecular outflow using 12CO (2���1) emission. Another class of low-luminosity objects has been identified thanks to Spitzer: Very Low Luminosity Objects (VeLLOs e.g., Young et al. 2004 Bourke et al. 2005 Dunham et al. 2006), some of which are found within evolved cores (as classified by Crapsi et al. 2005). These objects have low intrinsic luminosities (L 0.1 L ) and are embedded in a dense core (di Francesco et al. 2007). As VeLLOs have only recently been revealed by Spitzer (Dunham et al. 2008), it is not yet clear whether these are sub-stellar objects that are still forming or low-mass protostars in a low-accretion state. Broadband SED modeling of VeLLOs suggest that these sources can be explained as embedded young stellar ob- jects (YSOs) with a surrounding disk. In the case of IRAM 04191+1522 (hereafter IRAM 04191), continuum observations using the IRAM Plateau de Bure interferometer (PdBI) were in- terpreted by Belloche et al. (2002) as produced from the dense core���s inner part without the need for a disk. Recently, Maury et al. (2010) presented high-resolution PdBI observations toward a sample of 5 Class 0 sources to study the binary fraction in the early stages of star formation. Their sample includes two previously known VeLLOs: L1521-F and IRAM 04191. Dust continuum emission is detected toward both objects, which may arise from either a circumstellar disk or from the inner parts of the envelope. Lack of detailed modeling of the SED or visibilities in these sources makes it hard to distinguish between these two scenarios. This paper presents observations of L1451-mm, a low-mass core without any associated mid-infrared source in which we have detected compact thermal dust emission and a molecular outflow, along with models constructed to derive the properties of this object. In Section 2, we discuss previous observations of L1451-mm. Section 3 presents data used in this paper. In Section 4, we present the analysis of the observations and radiative transfer models to reproduce the observed SED and the continuum visibilities to constrain the physical conditions of the source. Finally, we present our conclusions in Section 5. 2. L1451-mm L1451-mm (also known as Per-Bolo 2 Enoch et al. 2006) is a cold dense core in the L1451 dark cloud located in the Perseus Molecular Cloud Complex. Here we assume that Perseus is at a distance of ���250 pc (Cernis 1990 Hirota et al. 2008), which is consistent with those used by previous works. L1451-mm is detected in 1.1 mm dust continuum with Bolocam at 31 resolution, and its estimated mass is 0.36 M from the Gaussian fit by Enoch et al. (2006) with major and minor FWHMs of 33 and 54 , respectively. However, the core is too faint to be identified by the SCUBA surveys at 850 ��m of the Perseus cloud (Hatchell et al. 2005 Kirk et al. 2006 Sadavoy et al. 2010). Figure 1 presents a summary of the observations pre-dating this work toward L1451-mm. Foster & Goodman (2006) pre- sented deep near-IR observations (J H Ks) of L1451-mm which show only heavy obscuration and no evidence for a point source. Establishing upper limits for this non-detection was complicated by the presence of extended bright structure (i.e., cloudshine) around the edge of L1451-mm. We estimate an upper limit by inserting synthetic stars with a range of magnitudes (in 0.1 mag steps) and appropriate FWHM at the central position. We ran Source Extractor (Bertin & Arnouts 1996) on these synthetic images using a 2.25 radius aperture and established the input magnitude at which a 3�� source was successfully extracted. This core is classified as ���starless��� by Enoch et al. (2008), because no point source is detected in Spitzer IRAC and MIPS images (J��rgensen et al. 2006 Rebull et al. 2007). Since the IRAC images do not contain significant extended emission we measured the flux in a 2.5 radius aperture centered on the central position of L1451-mm with a background annulus of 2.5���7.5 using the IRAF phot routine and applied the aperture correction factor for this configuration from the IRAC instrument handbook. All fluxes measured this way were within 2�� of zero (fluxes were both positive and negative). For MIPS we used the smallest aperture with a well-defined aperture correction factor, which is 16 . Both MIPS1 and MIPS2 were consistent with zero flux while MIPS3 was a weak (2.7��) detection. A summary of the photometric results is presented in Table 1. Given the lack of detectable emission at Spitzer wavelengths, and using the correlation between 70 ��m and intrinsic YSO luminosity determined by Dunham et al. (2008), an upper limit of L 1.6 �� 10���2 L on the luminosity of a source embedded within L1451-mm is determined. For a given SED, two quantities can be calculated to describe it: bolometric luminosity, Lbol, and bolometric temperature, Tbol. The bolometric luminosity is calculated through integration of the SED (S�� ) over the observed frequency range, Lbol = 4�� d2 S�� d��, (1) while the bolometric temperature is calculated following Myers & Ladd (1993), Tbol = 1.25 �� 10���11 �� Hz K, (2) where �� = �� S�� d�� S�� d�� . (3) For L1451-mm, if the upper limits are used as measurements, then we obtain Lbol 0.05 L and Tbol 30 K (see Dunham et al. 2008 Enoch et al. 2009b for discussions on the uncertainties in calculating Tbol and Lbol). This bolometric luminosity is lower than any of the Class 0 objects studied by Enoch et al. (2009b) in Serpens, Ophiuchus, and Perseus Molecular Clouds and also it is fainter than any of the VeLLOs with (sub-)millimeter wavelength observations studied by Dunham et al. (2008). J. E. Pineda et al. (2011, in preparation) present NH3 (1,1) and (2,2) line maps observed with the 100 m Green Bank Telescope (GBT). From these observations, they derive an almost constant (within a ��� 1 radius) kinetic temperature, Tkin ��� 9.7 K, and velocity dispersion, ��v ��� 0.15 km s���1, showing no evidence for heating from a central source. 2