A protocluster in Orion: OMC-2 FIR 4CHESS Key Programme studies with Herschel/HIFI and PdBI

A. Lopez-Sepulcre et al.: Dissecting an intermediate-mass protocluster

Fig. 3. Velocity-integrated PdBI contour maps obtained towards OMC-2 FIR 4, overlaid on the naturally weighted continuum map (grey scale).The bottom-left ellipse in each panel represents the beam size. Left: CH3OH(3,1,+0 – 2,1,+0) (black) and C34S(3–2) (magenta). Centre: DCN(2–1) (black) and DCO+(2–1) (magenta). All the contours start at 3! and increase by steps of 3!. The 1! RMS values are 0.16, 0.14, 0.17, and0.08 Jy beam!1 km s!1, respectively for the CH3OH, C34S(3–2), DCN(2–1), and DCO+(2–1) integrated maps. Right: Polygons representing thethree di!erentiated components, main, west, and south (see text), used to extract average spectra. The white crosses mark the positions of the threesources identified in this work (see Table 4).

bution from extended emission in these tracers which is filteredout by the interferometer.

Table 3. Flux loss for the PdBI molecular lines

Line Flux loss(%)

CH3OH 13DCO+(2 – 1) 73C34S(3 – 2) 60DCN(2 – 1) 72

Given the low spectral resolution of the WIDEX spectrum(1.95 MHz, i.e. 4 km s!1), it is not possible to analyse the shapeof the line profiles. However, this is beyond the scope of thepresent work, which aims primarily at investigating the spa-tial distribution of the emission in the di!erent tracers. To thisend we provide, for each millimetre molecular tracer, velocity-integrated maps in Fig. 3, and velocity channel maps in Fig. 4.The synthesised beam size of these images is 4.8"2.7 arcsec, andis depicted on the bottom-left corner of each panel in the figures.

If we focus first on the integrated maps (Fig. 3), the morpho-logical complexity of the source becomes evident from a com-parison of the emission distribution in the di!erent tracers:

- The CH3OH emission is dominated by the main source (seeSect. 3.1 above), which displays an elongated shape, andshows hints of a secondary component towards the west (theabove mentioned west source), mimicking the spatial dis-tribution of the 2-mm continuum emission. This morphol-ogy does not change when seen in the blended CH3OH linesaround 145 GHz.

- C34S(3–2) displays a more compact emission that traces boththe main and west sources.

- The DCN(2–1) emission peaks at the main source, but it ex-pands towards the west and the north, following the elon-gated morphology of the west source as seen in the contin-uum images.

- DCO+(2–1) peaks to the South of the main and west sources,a di!erent region which we name south source, not welltraced by any other line.

In summary, from the integrated maps we conclude thateach molecular line traces the region di!erently. We di!eren-tiate three spatially separated emission components, which wehave named main, west, and south sources, lying # 0.01 pc (i.e.2000 AU) apart from each other. These regions are schematicallydepicted in the right panel of Fig. 3 by three magenta polygonsthat have been manually defined to pinpoint similar central ar-eas for each while keeping approximately their morphologicalshapes. From these we have extracted the respective continuumand line fluxes for each tracer. The resulting fractional fluxeswith respect to the total flux of the entire region are listed inTable 4. The central coordinates of each source correspond tothe emission peaks of the 2-mm continuum image for the mainand west sources, and those of the DCO+(2–1) map for the southone. Notice that, with the exception of DCO+(2–1), due to therelatively small area of each polygon, the sum of fluxes is smallerthan the total flux in the whole region. This table reflects the pre-dominance of the main source’s flux over that of the other twofor all the tracers except DCO+(2–1), for which the south sourcedominates the overall emission.

Using Eq. 1, we have computed the mass, M, of each sourcefor two values of the dust temperature: 20 and 50 K. These arelisted in Table 5, together with the corresponding gas columndensities derived from the equation:

NH2 =M

µ mH " d2 (2)

where mH is the mass of the hydrogen atom, µ = 2.33 is themean molecular mass in units of hydrogen atom masses, and "is the solid angle.

The channel maps in Fig. 4 reveal more details about thespatial distribution of the molecular line emission. Despite thelow spectral resolution of the observations, the emission in allthe tracers but DCO+(2–1) is detected above 5! across 5 ve-locity channels (i.e. covering a width of about 18 km s!1), andtherefore some kinematical information can already be extractedfrom these maps. We note, however, that in the case of DCN(2–1) this velocity spread is partly due to the hyperfine splitting(see Sect. 3.2.3 below). In Fig. 5 we show the average spectrawithin the main, west, and south sources, defined by the poly-gons shown in Fig. 3 (right panel). Figure 6 presents the firstmoment maps for the four molecular tracers observed with the

6

In Brief

The HIFI spectral survey

Kinematics

Multiple cores

New foreground PDR

Line cooling

Plotting, for each species, the mean vlsr and FWHM, we find groups: outflow shock tracers are top right, multiple quiescent components appear at bottom.

High angular resolution observations with the Plateau de Bure Interferometer show clumpy substructure and chemical differentiation (see figures). Radio continuum observed with VLA could be an HII region -- if so, it’s driven by a B4 star.

Within 480 to 1901 GHz, lines contain 2% of all flux. Of the line flux, CO lines carry 60%, H2O lines 12% and CH3OH lines 9%. The role of SO is below that typical for CHESS protostars, and a factor 102 below Orion KL.

In a HIFI spectrum from 480 to 1901 GHz we detect 718 lines from 40 species, with an Eu range from 24 to 752 K and a diversity of line profiles.

60% of the lines are from CH3OH. No molecules more complex than methanol are seen.

One group of species is at 2 km/s blueshift. These are PDR tracers such as OH+, H2O+ and HF. An analysis gives nH=100cm-3, G0=1500 for this new PDR, which we propose to be an extended, tenuous foreground cloud within a few pc of OMC.

Kama et al. (2013, submitted)

Kama et al. (2013, submitted)

López-Sepulcre et al. (2013)

López-Sepulcre et al. (2013, submitted)

Kama et al. (2013, submitted); Ceccarelli et al. (2010)

3mm continuum (Shimajiri et al. 2008)The Orion constellation (left) and an IRAS map showing dusty cores (right)

OMC-2 FIR 4Orion

M.Kama*, A.López-Sepulcre, C.Ceccarelli, C.Dominik, E.Caux, A.Fuente et al.

Cecilia Ceccarelli

QUESTIONS addressed by the CHESS KP

What is the chemical composition during the first stages of star formation?

How does it influence the dynamical process and how it is influenced by the dynamical process?

What is the dynamical and physical structure during the early stages of star formation?

How all this depend on the mass of the forming star? On the evolutionary state?

ALMA&Herschel Garching 2010

* Mihkel KamaLeiden Observatory, Leiden University / University of Amsterdammkama@strw.leidenuniv.nl

OMC-2 FIR 4 is an intermediate-mass (~30Mo) protostellar core in Orion. We present line-rich Herschel/HIFI spectra and PdBI images which reveal new source components, allow to quantify the cooling budget, and more.FIR 4 may be forming multiple protostars.

1 - Institut de Planétologie et d’Astrophysique de Grenoble, Grenoble, France2 - API, University of Amsterdam, the Netherlands3 - CNRS/IRAP/Université de Toulouse, Toulouse, France4 - Observatorio Astronómico Nacional, Alcalá de Henares, Madrid, Spain