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Exploring the engines of molecular outflows

Leonardo Testi
e-mail: testi@arcetri.astro.it

Dipartimento di Astronomia e Scienza dello Spazio, Università degli Studi di Firenze, Largo E. Fermi 5, I-50125, Firenze, ITALY

Abstract:

Water vapour masers and CO outflows are well known to be associated with the youngest phases of evolution of massive stellar objects. Nevertheless, up to now there is a lack of high resolution multiwavelength study of the regions containing these objects. Using the VLA, the CSO and the TIRGO equipped with the new Near-Infrared (NIR) camera ARNICA, we have begun a systematic study of water maser/CO outflow regions. These new high resolution and high sensitivity data have proved to be very useful in probing the star formation activity and the connection between infrared and radio sources. Here we report the results obtained in a preliminary sub-sample of objects. The NIR data showed that both the maser spots and the large-scale outflows tend to be associated to the most embedded and probably younger sources of the infrared clusters. Infrared emission lines observed with narrow band filters show the presence of jet-like structures in most of the sources observed. Water masers, jet-like and Herbig-Haro-like infrared structures, and CO outflows enable to probe ejection phenomena at all spacial scales ranging from 0.01 to 1 parsec.

Contents

1. Introduction

masers and CO outflows are well known to be signposts of star formation, see also the contribution of Codella & Palla, this volume. Recent surveys have shown that the two events are usually associated (Felli et al. (1992)), a nd are related to the same Young Stellar Objects (YSO). On the other hand, these phenomena trace widely different physical scales, ranging from to 1 parsec. Which relationship exists between the two events and in which evolutionary stage of a YSO they are produced are key questions to address in order to understand the physical processes occurring during the star formation process. Only high sensitivity and high resolution line and continuum observations in the radio, submillimeter and infrared windows can hopefully address such questions.

Using the VLA in its most extended (A) configuration for maser line and GHz continuum high resolution mapping, the CSO for submillimeter molecular line emission observations, and the TIRGO for NIR imaging, we are carrying out a detailed study of a sample of regions with known masers and broad-wing CO lines. In this paper we will present the preliminary results obtained on two of the observed objects: BD (Palla et al. (1995)) and AFGL 5142 (Hunter et al. (1994)).

The first field (BD) is centered on a well known Herbig Ae/Be star, and the maser was discovered during a survey carried out by Palla & Prusti (1993) using the Medicina radio telescope for water vapour observations toward Herbig Ae/Be stars. AFGL 5142 is a well known dense molecular core (Verdes-Montenegro et al. (1989)), with no optically visible young stars detected. In spite of the supposed difference between the two regions our data reveal that the source powering the masers and the CO outflows is indeed very similar in the two cases and has the same infrared characteristics of sources detected toward a larger sample of masers (Testi et al. (1994)).

2. Observations

The observations will be described briefly in the following sections. For a complete description of the observations and data reduction see Palla et al. (1995) and Hunter et al. (1994). In particular, more information on ARNICA and on the NIR techniques used with TIRGO can be obtained here.

2.1. NIR observations

Near infrared images in the J(), H(), and K() broad bands were obtained at TIRGO, with the NIR imaging camera ARNICA, using a mosaicing technique. The observed fields cover roughly , with a scale of pixel, and a measured Point Spread Function of (Full Width at Half Maximum). In addition, for the AFGL 5142 field, narrow band images were obtained in the H () and in the Brackett () filters.

Photometric calibration was achieved by observing a set of UKIRT faint standard stars before and after the broad band observations. The narrow band images have been calibrated with the broad band ones, assuming that a set of stars does not have detectable line emission. The limiting magnitudes (3 in aperture) achieved in J, H, and K were , , and respectively, in both fields.

2.2. VLA observations

The VLA line and GHz continuum observations have been performed with the array in its most extended (A) configuration. The synthesized beam obtained were at 22 GHz and at GHz. The data were calibrated and reduced using the AIPS software.

2.3. CSO observations

The sources were mapped using the on-the-fly technique in various transitions of CO and CS, the beam was between and , depending on the transition. All the data have been reduced and analyzed using the CLASS software. The calibration is expected to be accurate within 20% and the pointing within .

3. Results

3.1. BD

  
Figure 1: NIR ``true-colour'' image of the BD field. The J-image is coded in blue, H- in green, and K- in red. See text for details.

  
Figure 2: On the left an overlay of the CO outflow map (blue wing, solid line; red wing, dashed line) on the K-band image greyscale, the position of the maser spot is marked with a triangle, the radio continuum source is coincident with BD (the saturated star to the north-west). On the right, (J-H,H-K) colour-colour diagram of the NIR sources the position of V1318S is marked.

In Figure 1 a NIR ``true-color'' image of the BD field is presented, the J-band image has been coded in blue, the H-band in green, and the K-band in red. In this kind of figure a Main-Sequence (MS) star should appear white-blue, all the orange and red sources are either suffer heavily from extinction or have a strong NIR excess. Sources with strong NIR excess have a Spectral Energy Distribution (SED) that falls toward longer wavelengths less rapidly than that of MS stars; in some extreme case the SED could rise toward longer wavelengths. Following Lada & Adams (1992) this kind of SED is characteristic of YSOs in which the young star is still embedded in an hot dusty envelope.

The VLA radio continuum map of these region contains only a faint pointlike object at the position of BD, while the line maps show only one maser spot roughly to the south-east. The maser spot is coincident with the star V1318S, which has a large NIR excess (see also Aspin et al. (1994)).

Our CSO observations were able to resolve the bipolar structure of the CO outflow in the region, whose presence had been inferred by the broad wings of the CO lines observed with poorer angular resolution. The outflow is very compact, the actual length is estimated to be less than , which correspond to pc. The dunamical center of the outflow is coincident with the maser spot, indicating that the very young embedded source V1318S is probably the source powering both these phenomena.

In Figure 2 an overlay of the CSO CO outflow map on the central region of the K-band image is presented; also shown in the figure are the locations of the maser spot and of the radio continuum source. On the right a (J-H, H-K) colour-colour diagram of the sources detected in the field is shown, the position of V1318S is marked. The colours of MS stars are shown with the continuum line, reddened MS stars should lie in the band in the ``reddening belt'' defined by the long-dashed lines. V1318S lies well on the right with respect to MS stars, indicating that it has a strong NIR excess.

3.2. AFGL 5142

In Figure 3 a NIR-true-colour image of the AFGL 5142 field is shown. A cluster of very red sources is detected at the center of the image, and the overlay with the CSO CO data (Figure 4) on the K-band map show that the NIR cluster is located at the dynamical center of the large outflow. A detailed look at Figure 3 shows that the red and the blue lobe centers do not coincide. We interpret this as the presence of two outflows in the region: a large-scale one () and a small scale one (), oriented almost perpendicularly. We identified a possible powering candidate for the large one in an infrared source designed IRS2, while the small-scale one is associated with a deeply embedded NIR source designed as IRS1. This picture is by our VLA and H molecular line observations. In the radio continuum a faint and compact source is detected at the location of IRS1 (most probably the free-free emission of a ionized stellar wind). The line observations reveal five maser spots, two of them roughly coincident with the continuum source, and the others aligned with the compact outflow axis. Comparing our results with that of Torrelles et al. (1992) a proper motion along this axis was marginally detected in the two most distant spots from the continuum source. In the H line a jet-like feature and several HH-like knots have been detected, the jet is pointing outward from IRS1 in the same direction of the red-lobe of the compact outflow. In Figure 5 an overlay of the VLA and the H data on the central region of the K-band image is presented.

  
Figure 3: Same as Figure 1 but for AFGL 5142.

  
Figure 4: Same as Figure 2 but for AFGL 5142. On the colour-colour diagram the positions of the NIR sources IRS1 and IRS2 are marked.

  
Figure 5: Overlay on the central region of the K-band image (greyscale) of the H data (after continuum subtraction, green contours), the VLA radio continuum peak (yellow concentric circles), and the VLA positions of the maser spots (violet crosses). The VLA position are determined much better than the dimensions of the markers. In red is the IRAS PSC error box.

The source IRS1 has been detected at with the JCMT (see the contribution of Jenness, indicating that is indeed a very young and deeply embedded YSO.

4. Conclusions

The source powering the masers and the compact molecular outflows shows very similar NIR properties, and appears to be YSO in the very preliminary stages of their lives. As discussed in Testi et al. (1994) the infrared properties of the objects appear to be those of young luminous stars deeply embedded in hot dust cocoons, responsible for the high extinction and for the NIR excess. In order to better understand the nature and evolutionary status of the sources, high sensitivity and high resolution mid- and far-infrared observations are needed. For this purposes the new ground-based mid-infrared instrumentation (like the new generation of TIRGO instruments: TIRCAM and TC-MIRC) as well as spacecraft mission (like ISO) are expected to produce new and very important results.

Acknowledgments

Partly based on observations collected at TIRGO (Gornergrat, Switzerland), VLA (Socorro, New Mexico, USA), CSO (Hawaii, USA). TIRGO is operated by CAISMI-CNR Arcetri, Firenze, Italy. This work has been done in collaboration with many people, among them I would like to acknowledge Marcello Felli, Gianni Tofani and Francesco Palla from the Arcetri Astrophysical Observatory and Todd Hunter and Greg Taylor from Caltech. I would like to thank the staff of the TIRGO, VLA, and CSO for help and hospitality. I would expecially thank Enrico Brunetti and Roberto Baglioni for helping with many technical problems.

References



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Wed Feb 22 22:36:08 GMT 1995