121 research outputs found
Star formation, structure, and formation mechanism of cometary globules: NIR observations of CG 1 and CG 2
Cometary globule (CG) 1 and CG 2 are "classic" CGs in the Gum Nebula. They
have compact heads and long dusty tails that point away from the centre of the
Gum Nebula. We study the structure of CG 1 and CG 2 and the star formation in
them to find clues to the CG formation mechanism. The two possible mechanisms,
radiation-driven implosion (RDI) and a supernova (SN) blast wave, produce a
characteristic mass distribution where the major part of the mass is situated
in either the head (RDI) or the tail (SN). CG 1 and CG 2 were imaged in the
near infrared (NIR) JsHKs bands. NIR photometry was used to locate NIR excess
objects and to create extinction maps of the CGs. The A_V maps allow us to
analyse the large-scale structure of CG 1 and CG 2. Archival images from the
WISE and Spitzer satellites and HIRES-processed IRAS images were used to study
the small-scale structure. In addition to the previously known CG 1 IRS 1 we
discovered three new NIR-excess objects, two in CG 1 and one in CG 2. CG 2 IRS
1 is the first detection of star formation in CG 2. Spectral energy
distribution (SED) fitting suggests the NIR-excess objects are young low-mass
stars. CG 1 IRS 1 is probably a class I protostar in the head of CG 1. CG 1 IRS
1 drives a bipolar outflow, which is very weak in CO, but the cavity walls are
seen in reflected light in our NIR and in the Spitzer 3.6 and 4.5 mum images.
Strong emission from excited polycyclic aromatic hydrocarbon particles and very
small grains were detected in the CG 1 tail. The total mass of CG 1 in the
observed area is 41.9 Msun of which 16.8 Msun lies in the head. For CG 2 these
values are 31.0 Msun total and 19.1 Msun in the head. The observed mass
distribution does not offer a firm conclusion for the formation mechanism of
these CGs: CG 1 is in too evolved a state, and in CG 2 part of the globule tail
was outside the observed area. (abridged)Comment: Accepted for publication in A&A. 22 pages, 24 figures. JHKs
photometry will be available electronicall
Rosette nebula globules: Seahorse giving birth to a star
The Rosette Nebula is an HII region ionized mainly by the stellar cluster NGC
2244. Elephant trunks, globules, and globulettes are seen at the interface
where the HII region and the surrounding molecular shell meet. We have observed
a field in the northwestern part of the Rosette Nebula where we study the small
globules protruding from the shell. Our aim is to measure their properties and
study their star formation history in continuation of our earlier study of the
features of the region. We imaged the region in broadband near-infrared (NIR)
JsHKs filters and narrowband H2 1-0 S(1), P, and continuum filters using
the SOFI camera at the ESO/NTT. The imaging was used to study the stellar
population and surface brightness, create visual extinction maps, and locate
star formation. Mid-infrared (MIR) Spitzer IRAC and WISE and optical NOT images
were used to further study the star formation and the structure of the
globules. The NIR and MIR observations indicate an outflow, which is confirmed
with CO observations made with APEX. The globules have mean number densities of
~. P is seen in absorption in the cores of
the globules where we measure visual extinctions of 11-16 mag. The shell and
the globules have bright rims in the observed bands. In the Ks band 20 to 40%
of the emission is due to fluorescent emission in the 2.12 m H2 line
similar to the tiny dense globulettes we studied earlier in a nearby region. We
identify several stellar NIR excess candidates and four of them are also
detected in the Spitzer IRAC 8.0 m image and studied further. We find an
outflow with a cavity wall bright in the 2.124 m H2 line and at 8.0 m
in one of the globules. The outflow originates from a Class I young stellar
object (YSO) embedded deep inside the globule. An H image suggests the
YSO drives a possible parsec-scale outflow. (abridged)Comment: 20 pages, 19 figures, accepted for publication in Astronomy and
Astrophysics, figures reduced for astro-p
Rosette Globulettes and Shells in the Infrared
Tiny, dense clumps of sub-solar mass called globulettes form in giant
galactic HII regions. The young central clusters compress the surrounding
molecular shells which break up into clumps, filaments, and elephant trunks
that interact with UV light from the central OB stars. We study the nature of
the infrared emission and extinction in the shell and globulettes in the
Rosette Nebula (RN) and search for associated newborn stars. We imaged the
northwestern quadrant of the RN in the near-infrared (NIR) through JHKs and
narrow-band H2 1-0 S(1), Pbeta and continuum filters. NIR images were used to
study the surface brightness of the globulettes and associated bright rims. NIR
photometry was used to create an extinction map and to search for NIR excess
objects. Archival images from Spitzer IRAC and MIPS 24 and Herschel PACS
observations were used to further study the region and its stellar population
and to examine the structure of the shell and trunks. The globulettes and
elephant trunks have bright rims in the Ks band on the sides facing the central
cluster. Analysis of 21 globulettes where surface brightness in the H2 1-0 S(1)
line is detected shows that about a third of the surface brightness observed in
Ks is due to this line: the observed average of the H2/Ks surface brightness is
0.26+-0.02 in the globulettes cores and 0.30+-0.01 in the rims. The estimated
H2 1-0 S(1) surface brightness of the rims is 3-8*10^{-8}
Wm^{-2}sr^{-1}um^{-1}. The H2/Ks surface brightness ratio supports fluorescence
as the H2 excitation mechanism. The globulettes have number densities of
n(H2)~10^{-4} cm^{-3} or higher. We confirm the results from previous optical
and CO surveys that the larger globulettes contain very dense cores and dense
envelopes, and that their masses are sub-solar. Two NIR protostellar objects
were found in an elephant trunk and one in the most massive globulette in our
study. (abridged)Comment: Accepted for publication in A&A. 24 pages, 27 figures. JHKs
photometry will be available electronicall
Near infrared imaging of the cometary globule CG12
Cometary globule 12 is a relatively little investigated medium- and low mass
star forming region 210 pc above the Galactic plane. NIR J, H, and Ks imaging
and stellar photometry is used to analyse the stellar content and the structure
of CG 12. Several new members and member candidates of the CG 12 stellar
cluster were found. The new members include in particular a highly embedded
source with a circumstellar disk or shell and a variable star with a
circumstellar disk which forms a binary with a previously known A spectral type
cluster member. The central source of the known collimated molecular outflow in
CG 12 and an associated "hourglass"-shaped object due to reflected light from
the source were also detected. HIRES-enhanced IRAS images are used together
with SOFI J,H,Ks imaging to study the two associated IRAS point sources,
13546-3941 and 13547-3944. Two new 12 micrometer sources coinciding with NIR
excess stars were detected in the direction of IRAS 13546-3941. The IRAS
13547-3944 emission at 12 and 25 micrometers originates in the Herbig AeBe star
h4636n and the 60 and 100 micrometer emission from an adjacent cold source.Comment: Accepted Astronomy and Astrophysics Oct. 15 200
Light and colour of cirrus, translucent and opaque dust in the high-latitude area of LDN 1642
We have performed a 5-colour surface photometric study of the
high-galactic-latitude area of dark nebula LDN 1642. Scattered light properties
are presented of diffuse, translucent and opaque dust over the range of 3500 --
5500 A. Far infrared absolute photometry at 200 um improves the precision of
and provides a zero point to the extinction. The intensity of the scattered
light depends on dust column density in a characteristic way: for optically
thin dust the intensity first increases linearly, then turns to a saturation
value; at still larger extinctions the intensity turns down to a slow decrease.
The value of the saturated intensity maximum shifts in a systematic way,
from 1.5 mag at 3500 A, to mag at 5500 A. The intensity
curves offer a straight-forward explanation for the behaviour of the
scattered-light colours. At the intensity peak the colour agrees with the
integrated starlight colour, while it is bluer at the low- and redder at the
high-column-density side of the peak, respectively. These colour changes are a
direct consequence of the wavelength dependence of the extinction. We have
compared the colours of the LDN 1642 area with other relevant observational
studies: high-latitude diffuse/translucent clouds, wide-field cirrus dust; and
externally illuminated AGB-star envelopes. For extragalactic
low-surface-brightness sources cirrus is an unwanted foreground contaminant.
Our results for cirrus colours can help to distinguish cases where a diffuse
plume or stream, apparently associated with a galaxy or a group or cluster, is
more likely a local cirrus structure.
Keywords: ISM: dust, extinction -- ISM: clouds, individual LDN 1642 --
Galaxy: solar neighbourhood -- Astronomical instruments, methods and
techniques: methods -- Physical data and processes: scatteringComment: 20 pages, 10 figures, accepted for publication in MNRAS, published as
MNRAS advance article on June 27 202
The structure of the cometary globule CG 12: a high latitude star forming region
The structure of the high galactic latitude Cometary Globule 12 (CG 12) has
been investigated by means of radio molecular line observations. Detailed, high
signal to noise ratio maps in C18O (1-0), C18O (2-1) and molecules tracing high
density gas, CS (3-2), DCO+ (2-1) and H13CO+ (1-0), are presented. The C18O
line emission is distributed in a 10' long North-South elongated lane with two
strong maxima, CG12 N(orth) and CG12 S(outh). In CG12 S the high density
tracers delineate a compact core, DCO+ core, which is offset by 15" from the
C18O maximum. The observed strong C18O emission traces the surface of the DCO+
core or a separate, adjacent cloud component. The emission in high density
tracers is weak in CG12 N and especially the H13CO+, DCO+ and N2H+ lines are
+0.5 km/s offset in velocity with respect to the C18O lines. Evidence is
presented that the molecular gas is highly depleted. The observed strong C18O
emission towards CG12 N originates in the envelope of this depleted cloud
component or in a separate entity seen in the same line of sight. The C18O
lines in CG 12 were analyzed using Positive Matrix Factorization, PMF. The
shape and the spatial distribution of the individual PMF factors fitted
separately to the C18O (1-0) and (2-1) transitions were consistent with each
other. The results indicate a complex velocity and line excitation structure in
the cloud. Besides separate cloud velocity components the C18O line shapes and
intensities are influenced by excitation temperature variations caused by e.g,
the molecular outflow or by molecular depletion. Assuming a distance of 630 pc
the size of the CG 12 compact head, 1.1 pc by 1.8 pc, and the C18O mass larger
than 100 Msun are comparable to those of other nearby low/intermediate mass
star formation regions.Comment: 18 pages, 17 figures Accepted A&A Sep. 22 200
Factor analysis as a tool for spectral line component separation 21cm emission in the direction of L1780
The spectra of the 21cm HI radiation from the direction of L1780, a small high-galactic latitude dark/molecular cloud, were analyzed by multivariate methods. Factor analysis was performed on HI (21cm) spectra in order to separate the different components responsible for the spectral features. The rotated, orthogonal factors explain the spectra as a sum of radiation from the background (an extended HI emission layer), and from the L1780 dark cloud. The coefficients of the cloud-indicator factors were used to locate the HI 'halo' of the molecular cloud. Our statistically derived 'background' and 'cloud' spectral profiles, as well as the spatial distribution of the HI halo emission distribution were compared to the results of a previous study which used conventional methods analyzing nearly the same data set
Star formation in Cometary globule 1: the second generation
C18O spectral line observations, NIR spectrosopy, narrow and broad band NIR
imaging and stellar J,H,Ks photometry are used to analyse the structure of the
archetype cometary globule 1 (CG 1) head and the extinction of stars in its
direction. A young stellar object (YSO) associated with a bright NIR nebulosity
and a molecular hydrogen object (a probable obscured HH-object), were
discovered in the globule. Molecular hydrogen and Br_gamma line emission is
seen in the direction of the YSO. The observed maximum optical extinction in
the globule head is 9.2 magnitudes. The peak N(H2) column density and the total
mass derived from the extinction are 9.0 10^21 cm-2 and and 16.7 Msun
(d/300pc)^2. C18O emission in the globule head is detected in a 1.5'' by 4'
area with a sharp maximum SW of the YSO. Three regions can be discerned in C18O
line velocity and excitation temperature. Because of variations in the C18O
excitation temperature the integrated line emission does not follow the optical
extinction. It is argued that the variations in the C18O excitation
temperatures are caused by radiative heating by NX Pup and interaction of the
YSO with the parent cloud. No indication of a strong molecular outflow from the
YSO is evident in the molecular line data. The IRAS point source 07178-4429
located in the CG 1 head resolves into two sources in the HIRES enhanced IRAS
images. The 12 and 25 micron emission originates mainly in the star NX Puppis
and the 60 and 100 micron emission in the YSO. The IRAS FIR luminosity of the
YSO is 3.1 Lsun.Comment: Language checked v2. Accepted for publication in A&A. 16 pages, 20
figures. C18O data will be available electronicall
C18O (3-2) observations of the Cometary Globule CG 12: a cold core and a C18O hot spot
The feasibility of observing the C18O (3-2) spectral line in cold clouds with
the APEX telescope has been tested. As the line at 329.330 GHz lies in the wing
of a strong atmospheric H2O absorption it can be observed only at high altitude
observatories. Using the three lowest rotational levels instead of only two
helps to narrow down the physical properties of dark clouds and globules. The
centres of two C18O maxima in the high latitude low mass star forming region CG
12 were mapped in C18O (3-2) and the data were analyzed together with spectral
line data from the SEST. The T_MB(3-2)/T_MB(2-1) ratio in the northern C18O
maximum, CG 12 N, is 0.8, and in the southern maximum, CG 12 S, ~2. CG 12 N is
modelled as a 120'' diameter (0.4pc) cold core with a mass of 27 Msun. A small
size maximum with a narrow, 0.8 kms-1, C18O (3-2) spectral line with a peak
temperature of T_MB ~11 K was detected in CG 12 S. This maximum is modelled as
a 60'' to 80'' diameter (~0.2pc) hot (80 K < Tex < 200 K) ~1.6 Msun clump. The
source lies on the axis of a highly collimated bipolar molecular outflow near
its driving source. This is the first detection of such a compact, warm object
in a low mass star forming region.Comment: APEX A&A special issue, accepte
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