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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
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)).
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.
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.
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.
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
.
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.
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.
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.