[PDF] 13CO and C18O J = 2–1 mapping of the environment of the Class 0

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A&A 545, A3 (2012)

DOI:10.1051/0004-6361/201219497

c?ESO 2012

Astronomy

Astrophysics

13

CO and C

18

OJ=2-1 mapping of the environment of the Class 0

protostellar core SMM 3 in Orion B9

O. Miettinen

Department of Physics, PO Box 64, 00014 University of Helsinki, Finland e-mail:oskari.miettinen@helsinki.fi

Received 27 April 2012/Accepted 24 July 2012

ABSTRACT

Context.Observations of molecular spectral lines provide information on the gas kinematics and chemistry of star-forming regions.

Aims.We attempt to achieve a better understanding of the gas distribution and velocity field around the deeply embedded Class 0

protostar SMM 3 in the Orion B9 star-forming region.Methods.Using the APEX 12-m telescope, we mapped the line emission from theJ=2-1 rotational transition of two CO isotopo-

logues, 13

CO and C

18

O, over a 4

×4 region around Orion B9/SMM 3.

Results.Both the

13

CO and C

18 O lines exhibit two well-separated velocity components at about 1.3 and 8.7 km s -1 . The emission

of both CO isotopologues is more widely distributed than the submillimetre dust continuum emission asprobed by LABOCA. The

LABOCA 870-μm peak position of SMM 3 is devoid of strong CO isotopologue emission, which is consistent with our earlier de-

tection of strong CO depletion in the source. No signatures of a large-scale outflow were found towards SMM 3. The

13

CO and

C 18

O emission seen at≂1.3 km s-1

is concentrated into a single clump-like feature at the eastern part of the map. The peak H 2 column density towards a C 18 O maximum of the low-velocity component is estimated to be≂10 22
cm -2 . A velocity gradient was found across both the 13

CO and C

18 O maps. Interestingly, SMM 3 lies on the border of this velocity gradient.

Conclusions.The

13

CO and C

18

O emission at≂1.3 km s

-1 is likely to originate from the "low-velocity part" of Orion B. Our analysis suggests that it contains high density gas (≂10 22
H 2 molecules per cm 2 ), which conforms to our earlier detection of deuterated species

at similarly low radial velocities. Higher-resolution observations would be needed to clarify the outflow activity of SMM 3. The

sharp velocity gradient in the region might represent a shock front resulting from the feedback from the nearby expanding Hiiregion

NGC 2024. The formation of SMM 3, and possibly of the other members of Orion B9, might have been triggered by this feedback.Key words.stars: formation - stars: protostars - ISM: clouds - ISM: individual objects: Orion B9/SMM 3 - submillimeter: ISM -

ISM: kinematics and dynamics

1. Introduction

The protostellar phase of low-mass star formation begins when a starless (prestellar) core collapses, and, after a hypothesised short-lived first-hydrostatic core stage (Larson 1969;Masunaga et al. 1998), a stellar embryo forms in its centre (the so-called second hydrostatic core; e.g.,Masunaga & Inutsuka 2000). The dense cores harbouringthe youngestprotostars are known as the Class 0 objects (André et al. 1993, 2000). In these objects, most of the system's mass resides in the dense envelope, i.e.,Menv M ,whereM is the mass of the central protostar. For this rea- son, Class 0 objects, or at least the youngest of them, are ex- pected to still represent the initial physical conditions prevailing at the time of collapse phase. Class 0 objects are characterised by accretion-poweredjets and molecular outflows, which can be verypowerfuland highlycollimated(e.g.,Bontempsetal. 1996; Gueth & Guilloteau 1999;Arce & Sargent2005,2006;Lee et al.

2007). The statistical lifetime of the Class 0 stage is estimated

to be≂1×10

5yr (Evans et al. 2009;Enoch et al. 2009), but

the exact duration of this embedded phase of evolution can be This publication is based on data acquired with the Atacama Pathfinder EXperiment (APEX) under programme 088.F-9311A. APEX is a collaboration between the Max-Planck-Institut für Radioastronomie, the European Southern Observatory, and the Onsala

Space Observatory.

highly dependent on the initial/environmental conditions (e.g.,

Vorobyov 2010).

The target source of the present study is the Class 0 proto- stellar core SMM 3 in the Orion B9 star-forming region, which was originally discovered by Miettinen et al. (2009; Paper I) through LABOCA 870-μm dust continuum mapping of the re- gion. SMM 3 is a strong submm emitting dust core (S 870

2.5 Jy) that is associated with a weakSpitzer24-μm pointsource

(S24 ?5 mJy), and a 3.6 Jy point source at 70μm. Using the Effelsberg 100-m telescope NH 3 observations, Miettinen et al. (2010; Paper II) derived the gas kinetic temperature of T kin =11.3±0.8 K in SMM 3. Using this temperature, the core masswasdeterminedtobe7.8±1.6M ,andits volume-averaged H 2 number density was estimated to be 1.1±0.2×10 5 cm -3 In the SABOCA 350-μm mapping of Orion B9 by Miettinen et al. (2012; Paper III), SMM 3 was found to be by far the strongest source in the mapped area (S 350
?5 .4Jy).Wealso found that it contains two subfragments, or condensations (we called SMM 3b and 3c), lying about 36?? -51 in projection from the central protostar. These correspond to 0.08-0.11 pc or≂1.7-2.3×10 4

AU atd=450 pc

1 . Because the thermal 1 In this paper, we adopt a distance of 450 pc to the Orion giant molecular cloud (Genzel & Stutzki 1989). The actual distance may be somewhat smaller as, for example, Menten et al. (2007) determined a trigonometric parallax distance of 414±7pctotheOrionNebula.

Article published by EDP SciencesA3, page 1 of10

A&A 545, A3 (2012)

Jeans length of the core isλ

J =0.07 pc, we suggested that the core fragmentation into condensations can be explained by thermal Jeans instability. Using the 350/870-μm flux density ratio, we determined the dust temperature of the core to be T dust =10.8 +5.7 -2.6

K, which is very close toT

kin within the error bars. The revised spectral energy distribution (SED) of the core yielded a very low dust temperature of 8 K, and a bolometric lu- minosityofL bol =1.2±0.1L . Thelatter is veryclose tothe me- dianluminosityofprotostarsin nearbystar-formingregions,i.e., L med =1.5 +0.7-0.4 L (Enoch et al. 2009;Offner & McKee 2011). In Paper III, we also studied the chemistry of SMM 3. We de- rived a large CO depletion factor off D (CO)=10.8±2.2, and a high level of deuterium fractionation, i.e., a N 2 D /N 2 H col- umn density ratio of 0.338±0.092. In Fig.1, we show the LABOCA 870-μm, SABOCA 350-μm, andSpitzer4.5/24-μm images towards SMM 3. In Table1, we provide an overview of the physical and chemical properties of SMM 3 derived in our previous papers.

In this paper, we discuss the results of our

13

CO and

C 18 O mapping observations of the environment of SMM 3. We analyse the structure of the mapped region as traced by emission from theJ=2-1 rotational transition of the above CO isotopologues. The rest of the present paper is organised as follows. Observations and data reduction are described in Sect. 2. Mapping results and analysis are presented in Sect. 3. In Sect. 4, we discuss our results, and in Sect. 5, we summarise and conclude the paper.

2. Observations and data reduction

The observations presented in this paper were made on

13 November 2011 using the APEX 12-m telescope located at

Llano de Chajnantor in the Atacama desert of Chile. The tele- scope and its performance are described in the paper by Güsten et al. (2006).An area of 4 ×4 (0.52 pc×0.52 pc atd=450 pc) was simultaneously mapped in theJ=2-1 rotational lines of 13

CO and C

18

O using the total power on-the-fly mode to-

wards SMM 3 centred on the coordinatesα

2000.0

=05 h 42
m 45.
s 8,

2000.0

=-01 16 13.

0[(+7.

5,+3.

0) offset from the SABOCA

peak position of SMM3]. At the 13

CO(2-1) and C

18

O(2-1) line

frequencies, 220398.70056 and 219560.357 MHz 2 , respec- tively,thetelescopebeamsize isabout28.

3 (HPBW). Thetarget

area was scanned alternately in right ascension and declination, i.e., in zigzags to ensure minimal striping artefacts in the final data cubes. Both the stepsize between the subscans and the an- gular separation between two successive dumps was 9.

4, i.e.,

about 1/3 times the beam HPBW ensuring Nyquist sampling. We note that the readoutspacing 1/3×HPBW should not be ex- ceeded to avoid beam smearing. The integration time per dump and per pixel was 1 s. As a frontend, we used the APEX-1 receiver of the Swedish Heterodyne Facility Instrument (SHeFI;Belitsky et al. 2007; Vassilev et al. 2008a,b). The backend was the RPG eXtended bandwidth Fast Fourier Transfrom Spectrometer (XFFTS; cf. Klein et al. 2012) with an instantaneous bandwidth of 2.5 GHz and 32768 spectral channels. The resulting channel separation,

76.3 kHz, corresponds to about 0.1 km s

-1 at 220 GHz. The telescope pointing accuracy was checked by CO(2-1) cross maps of the variable star RAFGL865 (V1259 Ori), and 2 The 13 CO(2-1) frequency was taken from Cazzoli et al. (2004), and it refers to the strongest hyperfine componentF=5/2-3/2. The C 18 O(2-1) frequency was adopted from the JPL spectroscopic database athttp://spec.jpl.nasa.gov/(Pickett et al. 1998). Fig.1.LABOCA 870-μm(top), SABOCA 350-μm(middle), and a SpitzerIRAC/MIPS two-colour composite image (bottom;4.5μmin green and 24μm in red) of the Class 0 protostellar core SMM 3 in OrionB9. TheLABOCAandSpitzerimages are shown withlinear scal- ing, while the SABOCA image is shown with a square-root scaling to improve the contrast between bright and faint features. The LABOCA contours, plotted in white, go from 0.1 (≂3.3σ) to 1.0 Jy beam -1 in steps of 0.1 Jy beam -1 . The SABOCA contour levels, plotted in green, start at 3σand are 0.18 Jy beam -1

×[1,2,4,6,8,10,12,14,16].

In the bottom panel, the first SABOCA contour, i.e., the 3σemission level, is plotted in white to guide the eye, and the white cross indicates the SABOCA peak position of SMM 3. The small subcondensations SMM 3b and 3c discovered in Paper III are labelled in the middle panel. Thegreen plus sign shows thetarget position of our previous molecular- line observations (i.e., the submm peak position of the LABOCA map before adjusting the pointing; see Paper III for details). A scale bar in- dicating the 0.05 pc projected length is shown in the bottom left of the top panel, with the assumption of a 450 pc line-of-sight distance. The effective LABOCA and SABOCA beams,≂20 and 10.

6, are shown in

the lower right corners of the corresponding panels. was found to be consistent within?4 . The focus was checked by measurements on Jupiter. Calibration was made by means of the chopper-wheeltechniqueandthe outputintensity scale given by the system isT ?A , which represents the antenna temperature corrected for the atmospheric attenuation. The amount of pre- cipitable water vapour (PWV) was in the range 1.28-1.48 mm,

A3, page 2 of10

O. Miettinen:

13

CO and C

18

O mapping of Ori B9-SMM 3

Table 1.Summary of the properties of SMM 3.

Parameter Value

SMM 3quotesdbs_dbs35.pdfusesText_40

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