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Multi Field imaging with the OVRO millimeter array
Leonardo Testi 1 and Anneila I. Sargent
California Institute of Technology, MS 105-24, Pasadena, CA 91125
Abstract. We review some of the basic guidelines used to plan, ex-
ecute and analyse multi-eld observations at the Owens Valley Radio
Observatory millimeter wave array. As a working example we discuss our
observations of the inner 5 0 .55 0 .5 of the Serpens cloud core. The results
of our observations of this proto-cluster core suggest that the stellar IMF
is determined during the core fragmentation process.
1. Introduction
High resolution imaging of extended areas of the sky at millimeter wavelengths
requires multi-eld interferometric imaging. Here we describe multi-eld imag-
ing at the Owens Valley Radio Observatory (OVRO) millimeter array, using as
a working example a recently completed project to map the Serpens cloud core
(Testi & Sargent 1998). We believe it is useful from the common user point
of view to describe a single coherent program from its inception through the
planning and acquisition of observations to the nal analysis and interpreta-
tion of the data. In addition, due to the specic goal of this particular project,
the observational and imaging techniques dier in important ways from those
presented by other contributors to this volume (e.g. Gueth and Plambeck).
Extended regions can be imaged at high resolution by combining maps of
contiguous elds in a single mosaic. As a general caveat for observers planning
to carry out such mosaic observations, we note that the widely (ab)used \rule of
thumb" that the sensitivity of an array for mosaicing observations is proportional
to nD, n being the number of antennas in the array and D the diameter of each
antenna, even if correct when considering various options during the design of a
new array, it may be very misleading when comparing dierent existing arrays.
In practice, the sensitivity is also a function of typical weather conditions at
the site, antenna and backend eôciencies, receivers noise, and, for continuum
observations, the available bandwidth. In general, the signal to noise ratio (S/N)
is proportional to the typical S/N for a single pointing synthesis image divided
by the square root of the number of pointing centers needed to cover the area to
be mapped. Thus the relative sensitivity for two dierent arrays is the ratio of
the single pointing S/N divided by the ratio of the diameters of the telescopes. In
addition, a uniform and ne sampling of the (u; v) plane is necessary to preserve
1 Osservatorio Astrosico di Arcetri, Largo E. Fermi 5, I-50125 Firenze, Italy
1

image delity. For an array with a limited number of antennas, several dierent
congurations must be employed.
2. Multi eld imaging at OVRO
The high surface accuracy of the six 10.4 m Leighton telescopes of the OVRO
millimeter array ensures very good antenna eôciency even at the highest ob-
servational frequency (Woody et al. 1998). The telescopes can be arranged in
dierent congurations along 455 m NS and 400 m EW tracks, yelding pro-
jected baselines from 10.4 m (the shadowing limit) to 484 m. Each telescope
is equipped with cryogenically-cooled SIS receivers operating simultaneously in
the 85-115 GHz and 205-265 GHz atmospheric transmission windows. Under
typical observing conditions, single sideband system temperatures at 100 GHz
are in the range 200-300 K. Spectral line observations employ a exible digital
correlator, which enables spectral resolutions from 0.016 to 4 MHz over band-
widths from 2 to 500 MHz. Simultaneous continuum observations are enabled
by an analog correlator with 1 GHz bandwidth per sideband. All antennas are
currently equipped with 22 GHz, slightly o-axis, receivers for real-time moni-
toring of the atmospheric water vapor line emission. These measurements are
used to correct for the atmospheric decorrelation (Woody et al. this volume).
A number of multi-eld imaging projects have been carried out succesfully
with the array over the last several years. These can be divided into two cate-
gories: (i) imaging of structures larger than the antenna primary beam size at
the relevant wavelengths, and (ii) maps encompassing signicant angular areas
of the sky. For the former, the sampling must always be ner than Nyquist, and
the pointing centers are typically spaced by 10-20% less than half the primary
beam FWHM (see e.g. Shepherd et al. 1998; 1999; Gueth and Plambeck, this
volume). Here we will concentrate on a project that falls with the latter category
and the sampling is somewhat coarser than Nyquist so that a larger area can be
mapped eôciently.
3. The OVRO proto-clusters project
There is now a fairly robust theory of how isolated, low-mass stars form (e.g.
Shu et al. 1987; 1993). One notable omission, is the failure to predict the
resulting stellar masses. Stars in the eld are known to be distributed according
to a well dened mass function (e.g. Salpeter 1955; Kroupa et al. 1993), and
a complete theory of star formation should explain this mass distribution, the
Initial Mass Function (IMF). Since it now seems very likely that most stars form
in clusters rather than in isolation (Clarke et al. 2000), the way in which the IMF
originates must be closely related to the way stars form in clusters. Indeed, young
embedded clusters appear to be populated by stars with a mass distribution very
close to that observed in the solar neighbourhood (Hillenbrand 1997).
The observed distribution of stellar masses may result naturally from the
protostellar accretion process (e.g. Adams & Fatuzzo 1996). Alternatively, it
may be caused by the fragmentation process in turbulent, cluster-forming, dense
cores (Myers 1998). If the fragmentation hypothesis is correct, the mass distribu-
tion of prestellar condensations in cluster-forming molecular cores should follow
2

the IMF of the stars in young embedded clusters and in the eld. Millimeter
wavelength interferometry oers both a high spatial resolution and high sensi-
tivity that are critical to shredding light on this problem. At 3 mm the thermal
emission from dust in the condensations is probably optically thin, allowing a
reasonable estimate of clump masses, and thus an insight into the clump mass
spectrum. In addition the OVRO array provides simultaneous observations of
molecular line and broad band continuum emission. Any contamination of the
continuum ux by molecular line radiation can therefore be eliminated. Finally,
thanks to the interferometric ltering capability, smooth, extended emission
from the molecular cloud core in which clumps are embedded is resolved out.
Using the OVRO array, we have begun a program of high resolution, millimeter-
wave mapping of molecular cloud cores with a view to establishing whether the
prestellar clump mass function and the IMF are in general similar. Here we
present our results for the Serpens star-forming core. Inside the 1500 M core is
a young stellar cluster of approximate mass 15-40 M (Giovannetti et al. 1998),
while far-infrared and submillimeter observations reveal the presence of a new
generation of embedded objects (Casali et al. 1993; Hurt & Barsony 1996). At
a distance of 310 pc (de Lara et al. 1991), and with an angular extent of few
arcmin (Loren etal. 1979), this is an ideal target to search for compact prestellar
and protostellar condensations.
4. Observations, Data Reduction and Imaging
For this project we centered our observing bands at about 99 GHz. This is a
compromise between dust emissivity and atmospheric optical depth, both in-
creasing with frequency. This region of the spectrum is also relatively free of
strong molecular line emission, except the CS(2{1) transition which falls in our
lower sidebandand and was observed with the digital spectrometer. The primary
beam FWHM of the OVRO antennas at 99 GHz is 73 00 . Since we were primar-
ily interested in surveying a large area of the sky for compact structures (i.e.
smaller that the primary beam), we maximized the eôciency by reducing the
number of pointing centers, and adopting a sampling pattern slightly coarser
than Nyquist. This is depicted in Figure 1, superimposed on published mil-
limeter continuum and CS(2{1) contour plots of the Serpens core. The pattern
comprises 50 separate pointings covering the inner 5:5 0  5:5 0 of the core.
Observations followed the standard OVRO procedure of interleaving 20
minute of on-source pointings and 5 minutes complex gain calibrator scans.
Passband calibration and ux density scale were established by observing strong
quasars (e.g. 3c273 and/or 3c454.3) and planets (Neptune and Uranus). Using
the in-house software package, MMA (Scoville et al. 1993), every \track" was
calibrated independently but employing standard techniques and the (u; v) data
were then combined together and written as standard FITS les for further pro-
cessing within AIPS. Two special precautions peculiar to mosaic observations
were taken: i) all pointing centers were observed in each track, and ii) all avail-
able array congurations were used. Both of these are critical to ensure optimum
quality in the nal images. As a result of observing all the pointing centers in
similar conditions, calibration uncertainties are averaged out and dierent parts
of the mosaic cannot be adversely aected by systematically dierent ux cal-
3

Figure 1. Left: grey dots show the pointing centers for the 50-eld
mosaic superposed on the 1.1 mm JCMT contour map and a near
infrared greyscale (Casali et al. 1993). Right: black circles centered on
the mosaic pointings are overlaid on a NRAO-12 m CS(2{1) contour
map (McMullin et al. 1994). Each circle is 10% smaller than the
OVRO primary beam FWHM.
C L E H
100 m
Plot file version 1 created 05­MAY­1998 22:15:09
V vs U for SM23 C1.DBCON.1 Source:SERPMAP
Ants ­* Stokes IF# Chn#
Kilo
Wavlngth
Kilo Wavlngth
­60 ­40 ­20 20 40 60
60
40
20
­20
­40
­60
Center at RA 18 27 21.998 DEC 01 12 30.00
SERPMAP BEAM 98226.459 MHZ RAWTA.IBEAM.1
PLot file version created 05­MAY­1998 22:22:41
Peak flux 1.0000E+00 JY/BEAM
Levs 1.0000E­02 ­10.0, 10.00, 25.00,
50.00, 75.00, 90.00)
ARC
SEC
ARC SEC
40 20 ­20 ­40
40
20
0
­20
­40
Figure 2. Top: schematics of the C, L, E, and H array OVRO con-
gurations. Bottom left: typical (u; v) coverage per pointing (for the
continuum). Bottom right: corresponding dirty beam contour levels at
10, 10, 25, 50, 75, and 90% of the peak.
4

Figure 3. The 3 mm continuum hybrid mosaic of the Serpens core.
The rms in the map is  0:9 mJy/beam, and the synthesised beam, in-
dicated by a black ellipse in the lower right corner, is 5:5 00 4:3 00 FWHM.
Submillimeter and far-infrared sources from Casali et al. (1993) and
Hurt & Barsony (1996) are indicated.
ibrations. The number of array congurations is important to maximize (u; v)
coverage. This is especially true in the case of an array with a relatively small
number of antennas, like OVRO, which provides only a limited number of base-
lines per conguration.
For the observations of the Serpens core, we employed four dierent con-
gurations of the array, H, E, L, and C. Schematical scaled representations of
these are shown in the top panel of Figure 2. In the bottom panels typical (u; v)
coverage per pointing after combining the continuum data from both sidebands
is displayed, together with the corrensponding dirty beam. Since Serpens is close
to declination 0 ô , strong north-south sidelobes are evident in the OVRO dirty
beam map.
The 3 mm continuum observations were deconvolved jointly using the AIPS
VTESS task. Because of the beam shape, and the presence of a few relatively
strong millimeter point sources in the eld, it was necessary to remove the
three brightest point sources from the data before deconvolution in order to
obtain a good cleaning. Cleaned images of the bright sources were then restored
into the mosaic after deconvolution. The resulting \hybrid" image is shown in
Figure 3. The noise level is  0:9 mJy/beam and the synthesised beam FWHM
5:5 00  4:3 00 . Previously detected far infrared and submillimeter sources from
Casali et al. (1993) and Hurt & Barsony (1996) are indicated by name. Our
more sensitive and higher resolution observations show several additional 3 mm
continuum sources (Testi & Sargent 1998).
5

Figure 4. The OVRO CS(2{1) integrated intensity contour map at
5 00 :54 00 :3 resolution. The location of the millimeter sources responsible
for known molecular out ows (SMM1, S68N, SMM3 and SMM4) and
the out ow orientations are represented by circles and arrows respec-
tively.
Each of the CS(2{1) pointings were deconvolved using the CLEAN-based
task IMAGR. The 50 clean cubes were then combined using the task LTESS.
We were unable to obtain a good joint deconvolution of the CS(2{1) cubes using
VTESS, mainly beacuse of the odd dirty beam shape. In Figure 4 we show the
resulting CS(2{1) integrated intensity image. Most of the extended emission
seen in the single dish map (Figure 1) is missing, probably because the optically
thick, extended, molecular line emission is resolved out by the interferometer.
Most of the CS(2{1) emission we detect appears to be due to the known out ow
sources (Testi et al. 1999). In Figure 4 we indicate these out ows along with
their approximate orientations (see also Wolf-Chase et al. 1998; Hogerheijde et
al. 1999).
5. Results
The mass of each of the dust condensations in Figure 3 can be calculated, as-
suming reasonable values for the parameters of the emitting dust (see Testi
& Sargent 1998 for details). Using existing near- and far-infrared observa-
tions (Givannetti et al. 1998; Hurt & Barsony 1996), the few condensations
associated with already formed stars were eliminated from our list of candi-
date prestellar and protostellar candidates. Figure 5 displays the cumulative
mass spectrum for the remaining 26 continuum sources above a the 4.5 peak
threshold of 4 mJy/beam. This mass distribution is well tted by a power law
6

Figure 5. Cumulative mass spectrum of the protostellar and prestel-
lar cores found in the 3 mm continuum mosaic shown in Figure 3.
The dotted line is the best tting power law dN=dM  M 2:1 , the
dashed line shows the Salpeter IMF, and the dot-dashed line depicts
the power law typical of gaseous clumps in molecular clouds, rejected
by the Kolmogorov-Smirnov test at the 98% condence level.
dN=dM  M 2:1 , signicantly steeper than the mass spectrum of larger-scale
gaseous clumps in molecular clouds dN=dM  M 1:7 (Williams et al. 2000),
which is rejected by the Kolmogorov-Smirnov test at the 98% condence level.
Our power law exponent, 2.1, is close to the Salpeter (1955) IMF value, 2.35,
and very similar to that suggested recently by Kroupa et al. (1993) for solar and
slightly subsolar stellar masses.
Our result support the ndings by Motte et al. (1998), their IRAM-30m
1.3 mm continuum maps of -Ophiuchi, another cluster forming core, show that
the mass spectrum of the prestellar and protostellar clumps within the core
appear consistent with the IMF (see also Andre this volume). Together these
provide compelling evidence for a \clump fragmentation" origin for the stellar
IMF. The conclusions are, however, preliminary, since the results of both surveys
may be vitiated by small number statistics. Combined millimeter and infrared
surveys of a larger number of cluster forming cores are necessary to ensure the
required statistical signicance. We note that in this respect next generation
millimeter arrays will be the ideal instruments to attain the required resolution
and sensitivity. In fact, at the low resolution attainable with single dish in-
struments, it is not possible to resolve the parent cores of each individual star,
and the mass distribution of the large scale gaseous clumps in molecular clouds
is recovered (see e.g. Tothill et al. this volume). For example, CARMA will
have both the required sensitivity and instantaneous (u; v) coverage to survey a
signicant number of regions eôciently (Mundy et al. this volume).
Acknowledgments. We thank P. Andre, P. Myers, D. Shepherd, and J.
Williams for useful discussions. Special thanks are due to the sta of the Owens
Valley Radio Observatory for their continuous and succesful eorts to provide
support and upgrades for the millimeter array. The Owens Valley millimeter-
wave array is supported by NSF grant AST-96-13717. Research on the formation
7

of young stars and planets is also supported by the Norris Planetary Origins
Project. Funding from the C.N.R.{N.A.T.O. Advanced Fellowship program and
from NASA's Origins of Solar Systems program (through grant NAGW{4030)
is also gratefully acknowledged.
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