PROPERTIES OF THE DUST IN THE MOLECULAR CLOUD TMC-2
CARLOS DEL BURGO & RENE LAUREIJS1
(1) EUROPEAN SPACE & TECHNOLOGY CENTRE (ESTEC), ASTROPHYSICS DIVISION, 2201 AZ NOORWIJK, THE NETHERLANDS
INTRODUCTION
Taurus molecular cloud complex is a nearby (d~140 pc)
low mass star forming region located at Galactic
latitude b~-16o and far from any OB association.
Large-scale maps in HI and CO, together with the
IRAS observations reveal highly-structured
morphologies (filaments, cores) of the atomic and
molecular gas and the dust in Taurus. Boulanger et al.
(1990, ApJ 364, 136) found large variations in the
IRAS color ratios I12/I100 on all scales within the
Taurus complex, and a correlation between I12/I100
and I60/I100. Abergel et al. (1994, ApJL 423, 59)
found that I60/I100 decreases dramatically from
diffuse to molecular clouds in Taurus and interpreted
it as result of very small grain variations.
Stepnik et al. (2003, A&A 398, 551) measured low temperatures (12.1
K, with power-law emissivity β=2) in the core of a filament, with the
envelope at 14.8 K and the large scale structure at 16.8 K. They
argued that those low temperatures can not be explained by radiation
transfer of the interstellar radiation field through the cloud. They
claimed the optical properties of the emitting grains have changed
and that grain-grain coagulation into fluffy aggregates takes place
inside the cold filament. The coagulation of grains has been also
proposed to be the cause of the enhanced far-infrared emissivity of
the big grains and the disappearance of very small grains toward
higher column densities in a sample of eight moderate density regions,
that includes the dark nebula LDN 1563 in Taurus (del Burgo et al.
2003, MNRAS 346, 403). For optical extinction Av>3.2 mag mantle
growth is observed (Whittet et al. 2001, ApJ 547, 872).
Fig. 1. Contours of 200 µm
ISOPHOT map overlaid on the
DSS2 blue image. The region is
centred at α2000=4h32m49.9s and
δ2000=24o23m35.5s . The C200
array detector (2X2 pixels;
92” pixel-1) is shown. Thick
circles mark the position of
the sources observed at 60 and
100 µm according to the IRAS
PSC catalog. Thin circles
correspond to those sources
only observed at 100 µm.
We present a study of a 31’x57’ region in Taurus (see Fig. 1).
The area contains TMC-2, some YSOs and few IRAS sources.
The motivation of this study was to extend the analysis of
del Burgo et al. (2003) to denser regions (AV,peak ~ 11 mag).
DATA PROCESSING
Observations were obtained with ISOPHOT (Lemke et al. 1996, A&A 315, L64), an instrument
on board of ESA’s Infrared Observatory (ISO, Kessler et al. 1996, A&A 315, L27). The AOT
PHT22 in raster mapping mode (37x20; step size of 92”) with the array detector C200 (2x2
pixels; 92” pizel-1) was used to map the region with the 120 and 200 µm filter-bands.
Data reduction was performed with the astronomical package PIA V10.0 (Gabriel et al. 1997,
Proc. ADASS VI Conf., 108). All standard signal correction steps (reset interval correction,
dark subtraction, signal linearization, glitch rejection) were applied. For flux calibration the
detector’s actual responsivity was derived from the FCS measurement obtained just after
the map. We used the first quartile normalization flat-fielding method of PIA to correct for
the remaining responsivity differences of the individual detector pixels.
We also use IRAS measurements (ISSA maps, Wheelock et al. 1994, ISSA Explanatory
Supplement, JPL Pub. 94-11) at 60 and 100 µm, DSS2 blue image, observations of 12CO
(J=1-0) (Dame, Hartmann & Thaddeus 2001, ApJ 547, 792), 13CO (J=1-0) (Mizuno et al. 1995,
ApJL 445, 161) and C18O (J=1-0) (Onishi et al. 1996, ApJ 465, 815), and a visual extinction
map obtained from 2MASS star counts (Padoan, Cambrésy & Langer 2002, ApJL 580, 57) of
the surroundings of TMC-2. ISSA IRAS 60 and 100 µm surface brightness calibration was
made consistent with the COBE/DIRBE photometric calibration system (see COBE/DIRBE
Explanatory Supplement 1997). The maps were created with the same angular resolution
(FWHM~4.2’, that of 100 µm IRAS maps) and with the C200’s grid. The zero levels of the
maps were made consistent with the zero level of Av using pixel-pixel correlations.
RESULTS
Far-infrared emission maps
Warm component
Correlations between FIR, cold HI and CO
Fig.2 shows the emission maps at 60 (top-left),
100 (top-right), 120 (bottom-left) and 200 µm
(bottom-right). It is observed a different
morphology in the 60 and 200 µm maps, that were
used to respectively trace the warm and cold
components of dust. The 100 and 120 µm
emissions were separated
into the warm and cold
components at these
wavelengths (see Fig. 2).
The resulting cold
components at 100 and
120 µm correlate better
with the 200 µm emission
(see Fig. 3); the warm
components at 100 and
Fig.3. Emission at 200 µm vr
120 µm with the 60 µm
cold component at 100 µm.
emission map.
The warm component has a temperature of 18.1 K in the
4.6’x9.2’ area around the peak emission at 60 µm (region A);
the temperature decreases to values of ~15 K for a samesize region 8’S from the peak (region B).
The optical depth at 200 µm (τ200=I200/Bν(T)) of the warm
component in regions A and B are only (1.0±0.1) 10-4 and
(2.2±0.4) 10-4, respectively.
Good correlations were found between 13CO (J=1-0) and the cold
emission at 100 µm (Fig. 6, left), with r=0.85, and for C18O with τ200
(Fig. 6, right) when considering an area around TMC-2 (r=0.93) and
the Northern area (r=0.84). We determined W(13CO)/Ic100 = (1.0±0.1)
K km s-1 MJy-1 sr, and W(C18O)/τ200= (122±17) 10-4 K km s-1 for TMC-2
and W(C18O)/τ200= (201±35) 10-4 K km s-1 for Northern. A certain
offset is observed in the C18O vs. τ200 for both regions.
60 µm
Cold component
Fig.4 (left) shows the colour temperature map of the cold
component. This is very uniform with a mean temperature
value of 12.5 K. The optical depth at 200 µm for the cold
component is in the range 15-70 10-4 (Fig. 4, right). Fig. 5
shows the ratios I200/Av and τ200/Av for independent pixels.
100 µm
Fig.6. Left: Ic100 (in MJy sr-1) overlapped to the 13CO map; right: τ200 (in 10-4)
overlapped to the C18O. The dashed line separates TMC-2 (south) and Northern.
N
100 µm, warm
CONCLUSIONS
100 µm, cold
Fig.4. Left: colour temperature map. Contour differences of 1 sigma
(0.3 K). Right: τ200 map. Levels are expressed in 10-4 units.
120 µm, warm
120 µm
120 µm, cold
200 µm
Fig.2. From top to bottom: emission maps at 60 µm (left) and
100 µm (right); warm and cold components at 100 µm (left
and right, respectively); warm and cold components at 120 µm
(left and right, respectively); emission maps at 120 µm (left)
and 200 µm (right). The circles correspond to IRAS sources
emitting at 60 and 100 µm . The 60 µm sources were removed.
Map scale and orientation are also indicated.
Fig.5. Top: I200/Av versus T for Taurus
(open circles). Filled circle the area of
TMC-2 with τ200 = 50 10-4. Errors bars
for few points are also indicated.
Bottom: τ200/Av versus T in Taurus.
Triangles correspond to the moderate
density regions observed by del Burgo
et al. (2003), filled squares correspond
to PRONAOS observations of a a dense
filament in Taurus (Stepnik et al.) and
MCLD123.5+24.9 in Polaris (Bernard et
al. 1999, A&A 347, 640), and big square
marks the position of the DISM. Also
values corresponding to the Thumb
Print Nebula (Lehtinen et al. 1998, A&A
315, L64) and LDN 183 (Juvela et al.
2002, A&A 382, 583) are marked with a
diamond and open triangle, respectively.
• Cold component presents a nearly uniform T=12.5 K
and has undergone a change in the dust grain properties
with respect to the DISM, in particular an increase in
the emissivity as indicated by τ200/Av.
• Warm component has a broad range of T’s with a
maximum of 19.8 K.
• Column densities derived from 13CO in good agreement
with Av. This supports that the high ratio τ200/Av is due
to a change (via coagulation and mantle growth) in the
FIR emissivity of the cold component.
• The high correlation between 13CO and Ic100 indicates
that the change in dust properties wrt DISM already
takes place at intermediate densities (n(H2)~103 cm-3).
•Also high correlation between C18O and τ200 for TMC-2
and Northern. Differences could be due to a gas
depletion in TMC-2.
CdB acknowledges Laurent Cambrésy and Tonikazy Onishi for providing the 2MASS extinction map and the 13CO and C18O molecular line maps of Taurus, respectively.
Presented at “Joint European and National Astronomical Meeting“, Granada (Spain), September 13-17, 2004. Contact address: cburgo@rssd.esa.int