Äîêóìåíò âçÿò èç êýøà ïîèñêîâîé ìàøèíû. Àäðåñ îðèãèíàëüíîãî äîêóìåíòà : http://www.arcetri.astro.it/~lt/preprints/g962f/g962.ps.gz Äàòà èçìåíåíèÿ: Tue Sep 11 16:56:52 2007 Äàòà èíäåêñèðîâàíèÿ: Sat Dec 22 07:00:37 2007 Êîäèðîâêà: Ïîèñêîâûå ñëîâà: massive stars

A&A manuscript no.
(will be inserted by hand later)
Your thesaurus codes are:
09.08.1; 09.09.1 G9.62+0.19; 08.06.2
ASTRONOMY
AND
ASTROPHYSICS
June 4, 2000
Detection of the thermal radio continuum emission from the
G9.62+0.19­F Hot Core
L. Testi 1 , P. Hofner 2;3 , S. Kurtz 4 , and M. Rupen 5
1 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, I­50125 Firenze, Italy
2 Physics Department, University of Puerto Rico at Rio Piedras, P.O. Box 23343, San Juan, Puerto Rico 00931
3 Arecibo Observatory, NAIC/Cornell University, HC3 Box 53995, Arecibo, Puerto Rico 00612
4 Instituto de Astronom'ia, UNAM, Apdo. Postal 70­264, 04510 M'exico D.F., M'exico
5 National Radio Astronomy Observatory, Socorro, NM 87801
Abstract. We present new high resolution and high sen­
sitivity multi­frequency VLA radio continuum observa­
tions of the G9.62+0.19­F hot molecular core. We detect
for the first time faint centimetric radio continuum emis­
sion at the position of the core. The centimetric continuum
spectrum of the source is consistent with thermal emission
from ionised gas. This is the first direct evidence that a
newly born massive star is powering the G9.62+0.19­F hot
core.
Key words: Hii regions -- ISM: individual G9.62+0.19 --
Stars: formation
1. Introduction
The formation of massive stars (M– 10 M fi ) has received
growing attention in recent years, because of their impor­
tant role in galactic evolution and the recognition that
the majority of low­mass stars are formed together with
high­mass stars in clusters (Clarke et al. 2000). One of
the earliest manifestations of a newly born massive star is
the appearance of an ultracompact (UC) HII region pro­
duced by the strong UV stellar radiation field. Since most
massive stars are formed in clusters it is expected that
other forming massive stars can be found close to UCHIIs.
Indeed, NH 3 (4,4) high angular resolution observations of
the molecular environment around UCHIIs (Cesaroni et
al. 1994) revealed compact (size ¸ 0:1 pc) and high tem­
perature (T kin ¸ 100 K) molecular clumps, so­called hot
cores (HCs). HCs are indeed close, but not generally co­
incident with, the UCHII (Cesaroni et al. 1994). Given
the high energy input required to maintain the HCs at
the observed temperature, they are likely to be heated by
young high mass stars. Since massive stars are expected
to reach the main sequence while still accreting (Palla
& Stahler 1993), the lack of centimeter radio continuum
emission at a few mJy level (e.g. Cesaroni et al. 1994)
can be explained in terms of the confinement provided by
Send offprint requests to: L. Testi; lt@arcetri.astro.it
the pressure of the hot molecular gas or by the infalling
material accreting onto the massive star (e.g. de Pree et
al. 1995; Xie et al. 1996; Walmsley 1995). Either of these
could effectively block the expansion of the ionised gas and
make the radio continuum emission extremely compact,
optically thick, and thus not easily detectable at cm wave­
lengths. HCs have been suggested to be sites of massive
star formation (Cesaroni et al. 1994; Kurtz et al. 2000).
However, if a young massive star is indeed present in­
side the HC and is heating the molecular gas, a region
of ionised gas should be present around the star, albeit
compact. The cm radio emission from these objects should
be optically thick and unresolved, with emission measures
exceeding 10 8 cm \Gamma6 pc.
At a distance of 5.7 kpc (Hofner et al. 1994),
G9.62+0.19 is a well known UCHII complex extensively
studied at high resolution in the centimetric radio contin­
uum (e.g. Garay et al. 1993; Cesaroni et al. 1994, among
others). Several HII and UCHII regions in different evolu­
tionary phases are present in the region, and there are in­
dications of a possible age gradient going from the western,
older, regions toward the eastern, younger, ones (Hofner
et al. 1994; 1996; Testi et al. 1998). The centimetric radio
continuum components have been designated from A to E
(Garay et al. 1993). High resolution thermal molecular line
and millimeter continuum observations revealed the pres­
ence of a HC in close coincidence with maser emission from
several different molecules and located midway between
radio components D and E. This new component, without
detected centimeter continuum emission but associated to
hot, ¸100 K, NH 3 , CH 3 CN and dust emission, was called
F (Cesaroni et al. 1994; Hofner et al. 1994, 1996). Inside
the molecular hot core a young massive star is presumed
to be forming (Hofner et al. 1996; Testi et al. 1998). If a
young massive object is indeed present within the HC, an
order­of­magnitude calculation suggests that the free­free
radio continuum emission at 22 GHz should be optically
thick with a total flux of ¸ 0:2--0:6 mJy and with a spatial
extent of ¸ 10 mas (Testi et al. 1998). We thus decided
to perform VLA high sensitivity radio continuum obser­

2 L. Testi et al.: Detection of the thermal radio continuum emission from the G9.62+0.19­F Hot Core
vations to detect the faint centimetric free­free emission
expected from such an object.
2. Observations
The G9.62+0.19 region was observed with the NRAO 1
VLA in the period May­June 1998 in the radio continuum
at 3.6 and 1.3 cm, and on 26 January 1999 at 0.7 and 2 cm.
The observing parameters are summarized in Table 1. The
2 cm dataset was obtained as a byproduct of the 0.7 cm
experiment: all antennas equipped with Q­band receivers
available at the time of the observations (12) were used
at 0.7 cm while the remaining 15 were employed at 2 cm.
At 0.7 cm we used a fast­switching observing cycle with
80 s on­source and 40 s on­calibrator (¸6 ffi away), which
resulted in a total switching cycle of ¸160 s and an ef­
ficiency of ¸50%. Hourly pointing sessions on the phase
calibrator at 3.6 cm were used to correct for pointing drifts
at 1.3 and 0.7 cm. 3C286 and/or 3C48 were observed to
set the flux scale, which is expected to be accurate within
10--15%.
All data editing, calibration and imaging were per­
formed within the AIPS software package. After stan­
dard flux and complex gain calibration, each dataset was
self­calibrated using one phase­only and one phase and
amplitude iteration. Consistency among maps at differ­
ent frequencies provided an internal consistency check of
our calibration procedures. Comparison of our fluxes with
previous observations for component D provided an addi­
tional check. At 0.7 cm heavy data editing was required
due to poor atmospheric conditions, only ¸12% of the
entire dataset was used to produce the final maps, corre­
sponding to the last ¸30 minutes of the run when atmo­
spheric fluctuations settled. All maps presented here have
been obtained using the AIPS IMAGR task with uniform
weighting of the visibilities and with the ROBUST pa­
rameter set to zero. In all cases we imaged an area at least
equal to the primary beam FWHM (see Table 1) to search
for emission. No correction for primary beam attenuation
has been applied at any frequency.
3. Results
In Figure 1 we show our radio continuum images at 3.6,
2.0, 1.3, and 0.7 cm of the region containing the known cm­
continuum components D--E (e.g. Cesaroni et al. 1994).
Components A, B, and C (Garay et al. 1993) are detected
in some of our maps depending on sensitivity and (u; v)
coverage, and are outside the shown area.
In addition to all the previously known cm­continuum
components, at 3.6 and 1.3 cm we detect four addi­
tional sources, labelled F to I, all above the 8oe level of
¸ 0:14 mJy/beam at 3.6 cm. At 2.0 cm only component F
1 The National Radio Astronomy Observatory is operated by
Associated Universities, Inc., under contract with the National
Science Foundation.
Table 1. VLA observing parameters
Phase center ff(J2000)=18 h 06 m 14.88 s
ffi(J2000)=\Gamma20 ffi 31 0 40.8 00
Parameter 3.6 cm 1.3 cm
Date 12May/05Jun98 15/17Jun98
Configuration A/BnA BnA
Time on source (hrs) 0.5/1.0 2.0/4.0
Freq./Bandwidth (GHz) 8.46/4\Theta0.05 22.46/4\Theta0.05
Flux cal 3C286/3C48 (Jy) 5.18/-- 2.5/1.17
Phase cal 1820\Gamma254 (Jy) 1.0 1.0
Primary beam (FWHP) 5. 0 4 2 0
Largest structure 10 00 5 00
Syn. beam (FWHM;pa) 0: 00 37 \Theta 0: 00 24;\Gamma5 ffi 0: 00 28 \Theta 0: 00 18;64 ffi
Noise (mJy/beam) 0.018 0.062
Parameter 2 cm 0.7 cm
Date 26Jan99 26Jan99
Configuration C C
Time on source (hrs) 4.5 2.5
Freq./Bandwidth (GHz) 14.94/4\Theta0.05 43.34/4\Theta0.05
Flux cal 3C286/3C48 (Jy) 3.43/-- 1.45/0.57
Phase cal 1820\Gamma254 (Jy) 0.87 0.86
Primary beam (FWHP) 3 0 1 0
Largest structure 30 00 10 00
Syn. beam (FWHM;pa) 2: 00 4 \Theta 1: 00 5;5 ffi 0: 00 68 \Theta 0: 00 64;\Gamma33 ffi
Noise (mJy/beam) 0.1 1.0
Table 2. Observed parameters of the newly detected sources
ff(J2000) ffi(J2000) F3:6cm F2:0cm F1:3cm F0:7cm
18 h 06 m \Gamma20 ffi 31 0 (mJy) (mJy) (mJy) (mJy)
F 14. s 884 39: 00 37 0.22 !0.5 0.62 !3
G 14. s 805 37: 00 17 0.15 0.7 a 0.39 !3
H 15. s 047 36: 00 72 0.98 0.4 0.36 !3
I 15. s 172 37: 00 97 1.0 1.6 0.8 !3
a ) The 2 cm position of source G is not exactly coincident with
that measured at 3.6 and 1.3 cm; the faint source is very close
to bright and extended sources, the flux cited is thus highly
uncertain due to possible imaging artifacts. Only higher angu­
lar resolution observations at 2 cm will offer a more accurate
position and flux.
is not detected. In Table 2, 3.6 cm peak positions and in­
tegrated fluxes or upper limits at each frequency for each
of the newly detected radio continuum components are
reported; all the newly detected sources are unresolved
by our synthesised beams. A detailed study of all the de­
tected sources goes beyond the scope of the present letter
and will be presented in a forthcoming paper.
In Figure 2 we show the position of the H 2 O and
OH masers (Forster & Caswell 1989; Hofner & Church­
well 1996), and of the mm­continuum component F
(Hofner et al. 1996) and our 3.6 cm (thin contours) and
the thermal NH 3 (5,5) (thick contours, Hofner et al. 1994)
maps overlaid on the 2.2 ¯m near infrared image from
Testi et al. (1998). Within the astrometric uncertainties
(Ÿ1 00 for the NIR data, Ÿ0.2 00 for all the other data), the
newly discovered cm­continuum source called F in Table 2
is coincident with the NH 3 (5,5) HC, the mm­continuum
and the NIR source.

L. Testi et al.: Detection of the thermal radio continuum emission from the G9.62+0.19­F Hot Core 3
Fig. 1. 3.6, 2.0, 1.3, and 0.7 cm radio continuum images of the G9.62+0.19D--I complex. The detected sources are labelled from
D to I (sources A, and C are outside the region shown, source B is only partially visible). At each wavelength, the synthesised
beam FWHM (see Table 1) is shown by the ellipses in the lower left corner of each panel.
Fig. 2. Our 3.6 cm continuum (thin contours) and the ther­
mal NH3(5,5) emission (thick contours, Hofner et al. 1994)
maps are overlaid on the 2.2 ¯m near infrared image of Testi
et al. (1998) of the G9.62+0.19D--F region. Filled triangles are
H2O masers (Hofner & Churchwell 1996), open squares OH
masers (Forster & Caswell 1989), the cross marks the position
of the mm continuum source F (Hofner et al. 1996; CH3CN
and mm­continuum emission is also found in coincidence with
sources D and E, but are not shown in this figure to avoid
confusion). The 3.6 cm contour levels are --50, 50 to 200 by
50 ¯Jy/beam, 0.3 to 0.9 by 0.1 mJy/beam, and 2 to 18 by
4 mJy/beam. The NH3(5,5) contours are --0.14, 0.14 to 0.3 by
0.77 Jy km/s/beam.
The HC and mm­component F located between the
cm­continuum sources D and E was the primary target of
our observations. We detected source F at 3.6 and 1.3 cm
and set upper limits at 2.0 and 0.7 cm. In Figure 3 we show
the radio continuum spectrum of source F. Data from the
present work are presented as filled circles, while the open
Fig. 3. Radio spectrum of component F. Filled circles: 3.6, 2.0,
1.3 and 0.7 cm this work; open circle: 1.3 cm from Cesaroni et
al. (1994); open square: 2.7 mm from Hofner et al. (1996). Up­
per limits are indicated by arrows. Top panel: spherical homo­
geneous UCHII region (long dashed line) plus thin dust emis­
sion (dotted line; fi=2). Bottom panel: spherical wind (short
dashed line) plus thin dust emission (dotted line; fi=1.5).
circle is from Cesaroni et al. (1994), and the open square
from Hofner et al. (1996).
4. Discussion and Conclusions
The primary goal of our new VLA observations was the
detection of radio continuum emission from the mm­
component and NIR source F, as predicted by simple con­
siderations in the case that a massive young stellar object
is hidden within and is heating the HC (Testi et al. 1998).
As shown in the previous section, one of our newly de­
tected cm­continuum sources is coincident with the HC.
In Fig. 3 we show the radio continuum spectrum of the
HC. The HC emission can be well fitted by a two compo­
nent model: free­free emission from ionised gas plus opti­
cally thin thermal dust emission at mm­wavelengths. The

4 L. Testi et al.: Detection of the thermal radio continuum emission from the G9.62+0.19­F Hot Core
presence of ionised gas requires a continuum source of en­
ergy either in the form of a UV photoionization field or
collisional ionisation. The detection of warm dust is con­
sistent with the presence of a very young massive star, as
implied also by the molecular gas observations (Cesaroni
et al. 1994; Hofner et al. 1996).
The ionised gas could be distributed either within
a very compact (r ¸ 0:0006 pc) spherical homogeneous
UCHII with EM¸3\Theta10 8 cm \Gamma6 pc, or in a spherical wind
(or r \Gamma2 density gradient) with F š / š 0:6 (Panagia &
Felli 1975), in either case the Lyman photon supply rate
could be provided by a zero age main sequence star ear­
lier than B1­B1.5 (Panagia 1973). This is in agreement
with independent estimates of the expected spectral type
derived from the molecular and infrared observations (O9­
B0.5; Hofner et al. 1996; Testi et al. 1998). The optically
thin thermal dust emission requires a dust emissivity in­
dex in the range fi=1.5­2.0.
Only two other HCs have been detected in the cm con­
tinuum W3(H 2 O) (Reid et al. 1995; Wilner et al. 1999)
and IRAS 20126+4104 (Hofner et al. 1999). In the first
case the radio emission is non­thermal from a synchrotron
jet, thus no constraint can be obtained on the nature of
the central (proto­)star. IRAS 20126+4104 has been de­
tected at one frequency only in the cm continuum, and the
mm spectrum is consistent with pure dust emission, while
the cm emission is most probably due to a jet, the exact
nature of which has not been firmly established yet. In the
case of G9.62+0.19­F the thermal origin of the emission
is confirmed by the radio spectral index constraint. The
source is unresolved in our ¸200 mas (¸ 1200 AU) beam,
while the two jets observed in the other cores, scaled to
the distance of G9.62+0.19, would have been marginally
resolved by our observations. Nevertheless, the possibility
of a thermal jet cannot be completely ruled out.
An alternative interpretation for HCs is that they
could be molecular clumps that are heated from the out­
side by a nearby star. This scenario is unlikely on theo­
retical grounds (Kaufman, Hollenbach and Tielens 1998)
and the radial temperature profiles which were measured
for three different HCs by Cesaroni et al. (1998) indicate
internal heating in those cases. In the case of G9.62+0.19--
F, our data provide plausible (but not decisive) arguments
for internal heating. First, the continuum source and the
peak of the molecular gas as given by NH 3 (5,5) coincide
to very high accuracy (better than 0: 00 3). This would not
necessarily be the case for external heating by an unre­
lated star outside the HC. Second, the measured bright­
ness temperature of the continuum of 42 K, together with
the indication of appreciable continuum optical depth im­
plies a very small linear size of the emitting region of about
120 AU. Thus, even if the object that is ionising source F
is external, it must be very nearby because the fraction
of ionising photons emitted into the solid angle subtended
by component F scales as \Gamma
D
r
\Delta 2
, where r is the distance to
the illuminating source and D the clump size of 120 AU.
For instance, at a distance of 1200 AU the required ZAMS
spectral type of the illuminating star would be O9.5 and
it is very unlikely that the effects of such a star on the
surrounding matter would remain undetected.
In summary, the presence of unresolved, optically
thick, thermal free­free emission is strong direct evidence
for a newly born massive star within the G9.62+0.19­F
HC. Thus, even within this complex cluster of UCHII re­
gions, the heating of the hot molecular gas within the HC
is most probably produced by an embedded young massive
star. This is consistent with the idea that the HC phase is
an evolutionary phase of young massive stars preceeding
the formation of an UCHII. However, in order to make
a firm statement in this respect, a larger sample of HCs
should be observed at several frequencies in the cm radio
continuum at high resolution and sensitivity.
Acknowledgements. We thank Riccardo Cesaroni, Marcello
Felli and the referee, Todd Hunter, for very useful comments
and stimulating discussion, and Barry Clark for nice schedul­
ing at the VLA. LT was partially supported by NASA Origins
of the Solar System program through grant NAGW­4030.
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