A star cluster at the edge of the Galaxy

Abstract

We study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789) through NIR (JHK), molecular line-, and dust continuum observations. The kinematic distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of about 20.2 kpc, this object is at the edge of the (molecular) disk of the Galaxy. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ca. 1.3 pc and an average stellar surface density of ca. 12 pc^{-2}. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded in a 1000 Mo molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is < 40 K, and the total mass of the swept-up material is < 10 Mo. Near the center of the flow, indications of much higher temperatures are found, probably due to shocks. A spectrum of one of the probable cluster members shows a tentative likeness to that of a K3III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud. Comment: Accepted for publication in A&A; 14 pages, 12 figures, 4 tables

Full text

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arXiv:astro-ph/0702541v1 20 Feb 2007
Astronomy & Astrophysics manuscript no. 5437
February 5, 2008
c ? ESO 2008
A star cluster at the edge of the Galaxy⋆
J. Brand1and J.G.A. Wouterloot2
1INAF - Istituto di Radioastronomia, Via P. Gobetti 101, I-40129 Bologna, Italy
2Joint Astronomy Centre, 660 N. A’ohoku Place, University Park, Hilo, HI 96720, USA
Received ;accepted
ABSTRACT
Context. This paper is part of our ongoing study of star formation in the (far-) outer Galaxy.
Aims. Our goal in this paper is to study stars and molecular gas in the direction of IRAS06145+1455 (WB89-789). The kinematic
distance of the associated molecular cloud is 11.9 kpc. With a galactocentric distance of ∼ 20.2 kpc, this object is at the edge of the
(molecular) disk of the Galaxy.
Methods. We use near-IR (J, H, K), molecular line-, and dust continuum observations.
Results. The near-IR data show the presence of an (embedded) cluster of about 60 stars, with a radius ∼ 1.3 pc and an average stellar
surface density ∼ 12 pc−2. We find at least 14 stars with NIR-excess, 3 of which are possibly Class I objects. The cluster is embedded
in a ∼ 1000 M⊙molecular/dust core, from which a molecular outflow originates. The temperature of most of the outflowing gas is
∼
<40 K, and the total mass of the swept-up material is∼
<10 M⊙. Near the center of the flow, indications of much higher temperatures
are found, probably due to shocks. A spectrum taken of one of the probable cluster members shows a tentative likeness to that of a
K3 III-star (with an age of at least 20 Myr). If correct, this would confirm the kinematic distance.
Conclusions. This cluster is the furthest one from the Galactic center yet detected. The combination of old and recent activity implies
that star formation has been going on for at least 20 Myr, which is difficult to understand considering the location of this object, where
external triggers are either absent or weak, compared to the inner Galaxy. This suggests that once star formation is occurring, later
generations of stars may form through the effect of the first generation of stars on the (remnants of) the original molecular cloud.
Key words. Stars: formation - Stars: pre-main sequence - ISM: clouds - ISM: individual objects: WB 89-789 (IRAS06145+1455)
1. Introduction
The H in our Galaxy extends out to galactocentric distances R
of at least 24-25 kpc (e.g., Wouterloot et al. 1990; McClure-
Griffiths et al. 2004). From high-sensitivity 21-cm observations,
Knapp et al. (1978) found H out to R ≈ 50 kpc (for a flat ro-
tation curve and R0=8.5 kpc). Molecular material and associ-
ated sites of star formationdo not appear at such large distances,
however. Star formation in the Galaxy has been observed to oc-
cur out to R ≈ 20 kpc (e.g., Fich & Blitz 1984; Wouterloot et
al. 1988; Kobayashi & Tokunaga 2000; Santos et al. 2000; Snell
et al. 2002). Wouterloot & Brand (1989) detected many molec-
ular clouds (hence star formation sites) towards IRAS sources
in the outer regions of the disk. Thus both star formation and
star formation reservoirs (i.e., molecular clouds) are present at
large distances from the Galactic center, but not as far out as the
atomic hydrogen. This suggests that beyond about R ≈ 20 kpc,
conditions are not favorable to transforming H into H2and then
into stars. The cause of this is not known (in general the for-
mation of H2clouds from H is not yet a settled manner, but it
must have to do with the physical environment presented by the
interstellar medium).
In the far-outer Galaxy (R > 16 kpc; hereafter FOG) the
physical environment differs from that in the inner Galaxy (for
a summary see Brand & Wouterloot 1995), the effects of which
may influence the formation of molecular clouds and the star
formation process within them. Also, both the volume density
of the molecular (and atomic) gas and the strength of spiral den-
sity waves are much reduced in the outer Galaxy. Even if star
Send offprint requests to: J. Brand (brand@ira.inaf.it)
formation is independent of cloud formation and is to be initi-
ated or enhanced by cloud-cloud collisions or triggered by su-
pernovae or density waves, then star formation activity in the
outer Galaxy would be expected to be significantly lower than
in the inner Galaxy. To study the process of star formation and
its end products in the outer parts of the galactic molecular disk,
we observed a selection of FOG clouds in the NIR to directly
detect the embedded stellar population. FOG clouds are at large
enough R for the physical conditions of the ISM to be different,
yet those in the 2ndand 3rdquadrants are close enough for the
star-forming cores to be resolved.
Here we present the most distant object studied with
these observations: WB89-789 (IRAS06145+1455; α2000
06h17m24.s2, δ2000=+14◦54′42′′) at (l,b = 195.◦82, −0.◦57),
which is located at R ≈ 20.2 kpc, d ≈ 11.9 kpc (Brand &
Wouterloot 1994; hereafter BW94). The luminosity of the IRAS
source (Lfir≈ 1.9×104L⊙) is consistent with a single star of type
B0.5V; it is embedded in a small (equivalent radius re≈ 5.7 pc,
and M ≈ 5.6 × 103M⊙[BW94]) cloud. An H2O maser was
detected towards this object (Wouterloot et al. 1993), indicating
that this is an active star-forming region. No radio continuum
emission was found within 4′of the location of the IRAS source
in our VLA A-array observations at 3.6 cm (Nov. 1993; unpub-
lished). With a 4σ detection limit of ∼ 3 mJy we would have
been able to detect an H region ionized by a B0.5V star or ear-
lier.
=

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2Brand & Wouterloot: Star cluster at the edge of the Galaxy
2. Observations and data reduction
2.1. Near-infrared
The data presented here were obtained on February 15, 1995,
with the ESO 2.2-mtelescope at La Silla (Chile). Images in J, H,
and K-bands were taken with the IRAC-2 camera and objective
C, which resulted in a scale of 0.′′49/pixel. The detector was a
256 × 256 pixel2NICMOS-III. The total instantaneous field-of-
view is therefore ∼ 2.1 × 2.1 arcmin2. During the observations
the seeing was 0.′′9.
From a quick K-band exposure (20×3 seconds) centered on
the WB 89-789IRAS point source position, a group of stars was
seen to be small enough in extent to allow an observing proce-
dure where it was placed at the center of each quadrant of the
CCD, as well as near the center of the CCD itself. Integrations
in the J-, H-, and K-bands were then made for 60 × 2 seconds,
resulting in 5 separate images per band. After correction with a
bad-pixel mask, each image was reduced separately by subtract-
ing a starless sky image derived by median averaging from the
other 4 exposures, and dividing by the flat field (see below). The
reduced images were then averaged to get the final frame, which
has an effective integration time of 10 minutes. Immediately af-
ter these “ON-source” observations a comparison (OFF-) field
was observed, at the same galactic latitude, but shifted towards
smaller longitudeby 0.◦2 (≈ 42pc at the distance of WB89-789).
To construct flat fields, dome exposures in all three bands were
taken at the start and at the end of each night.
We observed 5 standard stars during the night, at different
airmass. Each standard star was observed at 5 different positions
on the CCD (with the same observing pattern as used for our
object)inall3bands.Eachobservationwas reducedindividually
and the derived photometric zero points (ZPs) were averaged,
after which the ZP at the airmass of the WB89-789 observation
was obtained through linear interpolation.
The standard star observationsrevealeda non-uniformCCD,
with some quadrants“hotter”(i.e.,registeringmoreelectrons for
the same number of infalling photons) than others. This non-
uniformity will increase the random error in the photometry of
the science objects. In part this is accounted for by the average
ZP, as the standards were observed in the same way as the sci-
ence object, hence the average ZP partly takes into account the
differences between the quadrants.
The mosaiced images in J, H, and K were aligned and trun-
cated to show the same region of the sky. Stars were identified
in each frame with the IRAF task DAOFIND, and photometry
was performed through DAOPHOT, using a point-spread func-
tion determinedfrom 5 relativelybright and isolated stars. Fitted
stars were subtracted from the images, after which DAOFIND
was run again on the residual frame. This procedure was re-
peated until no more stars were found (usually after 2 or 3 it-
erations). Derived magnitudes were then corrected to take into
account the difference between the point spread fitting radius (3
pixels) and the aperture radius used (30 pixels) on the standard
stars to derivethe ZP; this correctionamounts to −0.m34, −0.m27,
and −0.m24 for J, H, and K, respectively.
2.2. Millimeter-lines
JCMT
On January 24, 2003, we used the 15-m James Clerk Maxwell
Telescope (JCMT) on Mauna Kea (Hawaii) to map the emission
of12CO(2–1) towards WB89-789 in the raster (on-the-fly) ob-
serving mode. The JCMT beam size at 230 GHz is 22′′; obser-
vations were made on a 10′′raster, using an autocorrelatorspec-
trometer at a velocity resolution of 0.2 kms−1. An off-position
was used at offset 600′′in right ascension from the IRAS source
position. The typical rms noise level in the spectra is 0.2 K (T∗
A).
On August 7 and 8, 2003, the JCMT was used to simulta-
neously observe13CO(2–1) and C18O(2–1) emission in a 80′′×
80′′region on a 10′′-raster towards WB89-789 using frequency-
switching over 8.2 MHz. The typical rms noise level in these
spectra is 0.13 K (T∗
simultaneous)13CO(2–1) observations of line calibrators shows
that our13CO(2–1) intensities must be corrected by multiplica-
tion with a factor of 0.88.
A). A comparison with separate (i.e., non-
On January 5, 2006, we used the JCMT to observe the
12CO(3–2)transitiontowardsWB89-789,inessentiallythesame
region covered by the12CO(2–1) observations. Observations
were made on a grid with 6′′spacing; the beam size at
345.796 GHz is 14′′. The velocity resolution is 0.136 kms−1,
and typical rms noise level is 0.6 K (T∗
beaccuratewithin1.′′8.Theline-datawerereducedandanalyzed
with the programs CLASS and GRAPHIC, which are part of the
GAG-software package developed by the Obs. de Grenoble and
IRAM-Grenoble.
A). Pointing was found to
IRAM 30-m
WB89-789 was observed in CS J=2–1, 3–2, and 5–4 with the
IRAM 30-m telescope at Pico Veleta (Granada, Spain) in the pe-
riod July 24 – 26, 1991. Maps were made of thirteen positions,
with 15′′spacing. The region mapped was sufficient to cover all
CS emission. The telescope beamwidth at these frequencies (98,
147,and245GHz)is respectively26′′,16′′,and11′′.Allintensi-
ties are on a T∗
channel filterbank of 100 kHz per channel, resulting in a reso-
lution of 0.31 and 0.20 kms−1, respectively; CS(5–4) was ob-
served with a resolution of 584 kHz (0.71 kms−1). Observations
were made by position switching against an off-position 30′W;
the typical rms noise level in the spectra at the central position
is 0.05 K. All spectra were reduced with the Grenoble CLASS
software.
Ascale. For CS(2–1) and (3–2) we used a 2×128-
2.3. Millimeter-continuum
JCMT
On April 10, 2002, we simultaneously observed the 450 µm and
850 µm emission towards WB89-789 with the Submillimetre
Common-User Bolometer Array (SCUBA; Holland et al. 1999)
at the JCMT. A standard 64-point jiggle map was made, which
covers an area 2.′3 in diameter; hence the observations are
limited to the region immediately around the IRAS source.
Calibration was performed by using fits to the time-dependence
of skydips and the nearby Caltech Sub-millimeter Observatory
(CSO) 225 GHz optical depth data, and average ratios of
τ(450 µm) and τ(850 µm) to τ(225 GHz). The final transi-
tion from instrumental parameters to Jy/beam was derived from
similar maps of Mars. The half-power beam size at 450 µm
and 850 µm are 9′′and 15′′, respectively. The rms values
in the WB89-789 maps are 450 mJy/beam (at 450 µm) and
20 mJy/beam (at 850 µm), respectively. The data were reduced
with the program SURF (Jenness & Lightfoot 1998). We found
that away from the continuum source, the background emission
level at both wavelengths was not around zero, but somewhat
higher. We have corrected this by subtracting 1 Jy and 0.05 Jy
from all pixel values in the 450 µm and 850 µm maps, respec-
tively.

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Brand & Wouterloot: Star cluster at the edge of the Galaxy3
SEST
Between August 17 and 22, 2003, the region around WB89-
789 was observed in the 1.2-mm continuumwith the 37-channel
bolometer array SIMBA (SEST IMaging Bolometer Array) at
the 15-m Swedish-ESO Submillimetre Telescope (SEST) at
ESO-La Silla, Chile. A region of size 600′′× 600′′(azimuth
× elevation) was scanned at a rate of 80′′/s; the total integration
time per map was about 12 minutes. Atmospheric opacity was
determined from skydips, which were taken every 2 hours; val-
ues at the zenith ranged between 0.15 and 0.45. The data were
transformed from counts/beam to mJy/beam by making similar
mapsofUranusonceperday.Theaverageconversionfactordur-
ing this observing period is 58±6 mJy/count.
In total, 8 maps were obtained. Each map was reduced and cal-
ibrated individually, after which they were averaged. The rms
in the final image is 15 mJy/beam. Pointing of the SEST was
checked by observing a strong continuum source every 2 hours.
The half-power beam width of the SEST at 1.2 mm is 23′′. Data
were reducedwith the programMOPSI, written byRobert Zylka
(IRAM, Grenoble), following the instructions from the SIMBA
Observers Handbook (Version 1.9, 9 Feb. 2003).
2.4. Spectroscopy
On December 7, 2004, we used the DOLORES spectrograph at
the Telescopio Nazionale Galileo (TNG) at La Palma (in service
observing-mode) to take a spectrum of one of the stars of the
cluster. Six separateintegrationsof 2220sec each were taken us-
ing the high-resolutionspectrographand HR-B Grism nr. 5 (dis-
persion 0.875Å per pixel), which resulted in a wavelength cov-
erage of about 3200Å-4900Å, and (with a 1′′slit) in a spectral
resolution of ∼3.2Å. Bias frames were taken on the same night,
andflat fields andHe andArcalibration-lampspectraweretaken
the night before and after. As a flux calibrator, Hiltner600 was
observed on the same night.
Data reduction was performed in IRAF, using standard pack-
ages. Bias frames were subtracted from all science and flat
field frames. An average normalized flat field was constructed,
which was corrected for bad pixels and then divided into the
star frames. Wavelength calibration was performed with the Ar-
lamp spectra, which in the wavelength range in question only
has 8 useful (i.e., non-blended) lines, none below 3780Å, and is
therefore not very accurate.
3. Results
3.1. Stars
3.1.1. Photometry, clustering, and pre-main sequence stars
InFig.1awe showtheK-bandmosaic,whilea false-color(JHK)
image of the area around WB89-789 (the region inside the box
in Fig. 1a) is shownin Fig. 1b. Just belowthe center ofthe image
we see a groupofstars embeddedinnebulosity;theIRAS source
is located in the northern half of the nebulosity. In this 1.′7 × 1.′7
area DAOFIND detects 173, 179, and 161 stars in the J-, H-, and
K-bands, respectively. The distributions of these stars in bins of
1 magnitude peak at mJ≈ 20 (faintest star found 19.96±0.24),
mH≈ 18 (19.59±0.54),and mK≈ 17 (18.52±0.50),respectively.
Signal-to-noise ratios are reached of 5–10, i.e., photometric ac-
curacy of 0.m1–0.m2, at mK= 16.5 − 17.5, which corresponds to
main sequence [ms] spectral types A0–5 (2–3M⊙) for a distance
of 10 kpc and visual extinction AV=10 mags.
Fig.2. Comparison between the star densities in the WB89-789
field (drawn) and in a field centered at a nearby off-position
(dashed).StarswerecountedontheK-imagesinconcentricrings
of equal area. In the WB89-789 field the rings were centered on
the nebulosity (Fig. 1b). An excess of stars in the WB89-789
field is clearly visible.
To confirm that WB89-789 is associated with a clustering of
stars, in Fig. 2 we compare the density of stars in the WB89-
789 field with that at a nearby off-position (see Sect. 2). Stars
were counted in concentric rings of equal area (π×15(pixels)2).
For the WB89-789 field the rings were centered on the nebulous
region visible in Fig. 3, and the counting was performed in the
boxed-in area of the K-frame shown in Fig. 1a. There is a clear
excess of stars compared to the background out to about 22′′
from the center of the nebulosity; this corresponds to a radius
of 1.3 pc, in which the average stellar surface density is 12 pc−2
(compared to ≤3 pc−2for the OFF-field). There is a particularly
strong concentration of stars in the inner 15 pixels (∼ 7.′′5); in
this region,with r ∼ 0.4 pc, the stellar surface density is 33 pc−2.
The central 0.′9 × 1.′1 part of the region shown in Fig. 1b is
where all 5 individual ON-source images overlap; a (K-frame)
contour plot of this region is shown in Fig 3. Sixty-eight stars
were detected in all three (J, H, K) bands. The photometric data
arecollectedinTable4.The(J–H),(H–K)-diagramofthesestars
is showninFig. 4. Thedottedlines definethe reddeningbandfor
normal stellar photospheres (Rieke & Lebofsky 1985). Objects
outside and to the right of this band have intrinsic NIR excess,
and for a number of them their location in this diagram can be
explained by the presence of circumstellar disks, with central
holes of various dimensions (see Lada & Adams 1992); these
are likely pre-ms stars.
There are 4 stars outside the normal reddening region on the
left(nrs.87,53,51,and20).All oftheseexceptnr.87areconsis-
tent with a location inside the normal region, considering error
bars. On the other hand, stars with effective temperatures be-
tween 2500 K and 4000 K are predicted to have an excess of
emission in the H-band, which is due to the formation of H2and
absorptionbyH−(Gingerich& Kumar1964); this will shiftstars
likenr. 87to theleftofthenormalreddeningregioninFig.4 (see

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4Brand & Wouterloot: Star cluster at the edge of the Galaxy
Fig.1. a (left). K-band image of the region around WB89-789 (IRAS06145+1455).The area of sky visible here is ∼ 3×3 arcmin2.
The region outlined by the black box has a size of ∼ 1.7× 1.7 arcmin2, and is shown on the right. North is up, East is left. b (right).
False-color image (J=blue, H=green, K=red) of a ∼ 1.7 × 1.7 arcmin2region around WB89-789
also Chini et al. 1992). There are 22 stars outside the normal re-
gion on the right. Eight (nrs. 7, 9, 13, 40, 63, 81, 98, 99) have
anomalous colors, in that they are also below the unreddened
TTau locus; these are indicated with circles in Fig. 6. Three of
these (nrs. 13, 40, 98) could be brought into normal region con-
sidering their error bars. The other 5 have ’anomalous’ colors,
which could be due to unresolved binaries (Lada et al. 2000).
That leaves at least 14 (22-8) stars with ’true’ NIR excess (i.e.,
21% of the total of 68); these are indicated with a star in Fig. 6.
Eleven are in the Class II-source region (between the right-hand
normal reddening line and the right-most TTau reddening line).
Their SEDs are dominated by disk emission and their location
can be explained by variation in size of central hole and incli-
nation (see Lada & Adams 1992). Three (nrs. 15, 54, 83) are in
the region of Class I sources. These are the most embedded and
youngest objects. Their SED is dominated by emission from an
envelope of gas and dust.
3.1.2. Extinction
The amount of visual extinction towards WB89-789 is de-
rived by means of a K versus (H-K) color-magnitude diagram
(Fig. 5a), plotting only those stars that are within 7.′′5 from the
center of the nebulosity and thus presumably part of the cluster.
Here we can also use those (7) stars that were detected only in
H and K, but not in J. In addition to the stars, we have drawn
the location of the unreddened main sequence for a distance of
11.9 kpc. Ignoring the stars that have been identified as having
a NIR-excess in Fig. 4, because they do not follow the standard
interstellar reddening law, we shift the main sequence along the
direction of the interstellar reddening (indicated by the arrow in
Fig. 5: AK = 0.112 × AVand E(H–K) = 0.061×AV; Rieke &
Lebofsky 1985) until it reaches the locations of the cluster stars.
Thus we derive a maximum amount of foreground visual ex-
tinction of AV≈ 6.25 magnitudes (maximum, because even the
least-reddened stars can have some amount of internal extinc-
tion, i.e., due to the dust in the molecular cloud in which they
are embedded). This may not seem much for such a supposedly
distant object, but it is consistent with the findings of, e.g., Fich
(1984) and Amˆ ores & L´ epine (2005), who concluded that the
extinction is fairly low (∼ 3 mags.) in much of the outer Galaxy,
and that there does not appear to be a progressively increasing
extinction beyond a few kpc from the Sun.
In Fig. 5b we show all 68 stars for which we have J, H, and K, as
well as the 15 stars for which only H and K are available. Again
ignoring stars with NIR-excess (identified with a large open cir-
cle), as well as the stars for which we only have H and K (for
which we cannot say whether they have NIR-excess) we see that
there are stars with an additional extinction of up to ∼ 30 mag-
nitudes, although the bulk has AV ∼
for stars inside and outside the above-defined cluster radius. As
we mentionedin the Introduction,the edgeof the moleculardisk
of the Galaxy lies at about R ≈ 20 kpc, thus it is unlikely that in
our images of WB89-789 there are any stars more distant than
the cluster, andwe concludethat most stars shownare part of the
cluster. Potentially the earliest-type star in the cluster that does
not have NIR excess is star 33 (see Fig 3): If one were to simply
shift it back to the the main sequence, it would be of spectral
type B0, consistent with the spectral type derived from the Lfir
(and assuming it is an ms-star).
<7.5. This is about the same
Figure 6 shows a section of the K-band image of the WB89-
789 area, with the contours of integrated C18O(2–1) emission
(see Fig. 7c) superimposed. The 14 stars with NIR-excess are
shown in Fig. 4b, and the 8 stars with anomalous colors are
marked in Fig. 6. Rather than being concentrated in the center
of the cluster (which we take to be the nebulous region seen in
Fig. 3), the NIR-excess stars are distributed in a ring around the
peakofthe C18O(2–1)emission.At thelocationofthemolecular
peak, no stars are seen in this K-image, which may be because
the extinction there is too high. The average column density of
H2, N(H2), derived from the C18O(2–1) data (see Sect. 3.3.1) is
7.8×1021cm−2. Because N(H2)=1021×AV(Bohlin et al. 1978,
and assuming the gas-to-dust ratio in the FOG is the same as lo-
cally) this implies a visual extinction AVof about 8 magnitudes
due to the whole of the molecular clump (front to back). At the
peak of the C18O(2–1) emission N(H2)≈ 1.0 × 1022cm−2, and
thus AV ≈ 10 magnitudes. If the cluster stars are embedded in
the molecular gas, as they are likely to be, then the general (i.e.,
not counting that which is due to the circumstellar material) in-
ternal extinction would therefore be on average ∼5 magnitudes
(and in any case ≤10 magnitudes), consistent with the number
derived above from the color-magnitude diagram.

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Brand & Wouterloot: Star cluster at the edge of the Galaxy5
Fig.3. a. Contour plot (K-frame) of that part of the final mosaic where all 5 ON-source images overlap. Sixty-eight stars in this
area have been detected in all three bands (J, H, and K). Several stars are labeled, to identify the field in panel b. Among them is
star 33, of which a spectrum has been taken (Sect. 2.4). North is up, East is left. The (0,0)-offset indicates the nominal position of
the IRAS source.
b. The cluster field from panel a (inside the box) shown in a larger context on the POSS II-R image.
Fig.4. a. NIR color-color diagram for the 68 stars in Fig. 3a that have reliable detections in all three (J, H, and K) bands. The thick
and thin drawn curves are the unreddened main sequence and giant branch, respectively, from Bessell & Brett (1988); the dashed
line indicates the loci of the classical T Tauri stars (Meyer et al. 1997); the dotted lines indicate the direction of normal interstellar
reddening (Rieke & Lebofsky 1985). Crosses on these lines mark increments of 5 magnitudes of visual extinction from the points
where they intersect the main sequence curve. Objects outside and to the right of the reddening band have intrinsic NIR excess; to
avoid confusion, error bars are shown only for objects that are well outside the normal reddening band. The filled triangle identifies
star 33 (see text and Fig. 3). b. Like a, but with the stars identified by their ID-number.
3.2. Molecular cloud
3.2.1. Cloud parameters
Whole cloud
BW94 mapped the molecular cloud associated with WB89-
789 in CO(1–0) with the SEST (beam size 45′′), on a 40′′raster.
We have obtained new, better sampled CO(2–1) and (3–2) maps
with the JCMT; the latter is shown in Fig. 7a. From the CO(2–1)
data, following the procedures outlined in BW94, we derive a
mass MCO ≈ 4.5 × 103M⊙(from N(H2)=X
?
T∗
R(CO)dv, with
X = 1.9 × 1020cm−2(Kkms−1)−1and using Tmb=
ηfss = 0.80 is the forward efficiency). The virial mass Mvir ≈
5.1 × 103M⊙ (Mvir = 126re(∆V)2for an r−2density distri-
bution; MacLaren et al. 1988); ∆V is the FWHM line width,
and the equivalent, beam-corrected radius re ≈ 5.3 pc. These
numbers agree very well with those derived by BW94 from
the CO(1–0) map. We also note that the virial mass and the
mass derived from the empirical method, using X, are in ex-
cellent agreement. The value of X quoted above is the inner
Galaxy value; Brand & Wouterloot (1995) have shown that in
the outer Galaxy X may be somewhat larger, but lies within
?
T∗
A/ηfssdv;