Oblaci u oblacima

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Oblaci u oblacima molekulski gas u Magelanovim
oblacima
Silvana Nikolic Astronomska opservatorija,
Beograd
image NOAO/AURA/NSF
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S3MC mosaic for the Spitzer IRAC 8.0, 4.5 and
3.6?m (RGB), Bolatto et al. 2007
SMC distance mod. 18.93/-0.024mag mean
metal. -0.64/-0.04 Fe/H KellerWood (2006) but
also -1.2Fe/H Van den Bergh (2006)
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Meixner et al. 2006
LMC distance mod. 18.54/-0.018mag
mean metal. -0.34/-0.03Fe/H dex KellerWood
(2006) but also -0.6Fe/H dex Van den Bergh
(2006)
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N88
N66
30Dor-10
SMC-B11
N159-W
Lirs 36
Lirs49
Hodge 15
N159-S
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X2x1020 cm-2 K km/s
H2? XN(H2)/I CO(1-0)

• correlation CO-Av extrapolation N(H)/Av diffuse
  ISM (Savage et al. 1977)
• excitation analysis 13CO, and the 12CO/13CO
  known
• virial analysis of the clouds, line widths and
  cloud sizes
• g ray emission (StrongMattox 1996)

ICO(1-0) gt N(CO) not trivial cannonical dark
N(CO)/N(H2)10-4 (Dickman 1978) but
diffuse translucent 4x10-7, 9x10-6 (Burgh et
al. 2006)
! sensitivity CO photodissociation to IS UV
radiation field, cloud geometry, UV absorption
and scattering properties of the dust (Van
dishoeckBlack 1988, Kopp et al. 2000)
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CO surveys LMC 1988, Cohen et al. Columbia
1.2m _at_115GHz HPBW 8.'8 ESO/SEST key
project 1992-1995 1999, Fukui et
al., NANTEN I 4m, _at_115GHz HPBW 2.'6 SMC -
1991, Rubio et al. Columbia 1.2m
ESO/SEST Key Project 1992-1995
2001, Mizuno et al. NANTEN I
CO in the Magellanic Clouds Israel et al.
(1993), ... Rubio et al. (1996), Lequeux et al.
(1994), ... Johansson et al. (1998), ... Israel
et al. (2003).
SEST 1987-2003, 15m 70-356GHz
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observations SMC CO(1-0), CO(2-1), 13CO(1-0),
13CO(2-1), Rubio et al. (1996) , new
CO(3-2) LMC -II- Johansson et al. (1998)
Tsys1000, 180, 150K _at_ 345, 146 and
98GHz resp. ?mb0.74, 0.66, 0.33 _at_ 100,
147 and 345GHz resp. HPBW 50, 34 and 15
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RADEX a non-LTE excitation and radiative
transfer code (Jansen et al. 1994)
4 the radaitive transfer equations the mean
escape probability (MEP)
approximation. collisionsspontane
ousstimulated radiative transitions
computes statistical equilibrium for rotational
levels of IS molecules 4the internal
radiation field (incl. 325 transitions, 26
levels) InbBn(TCBG)(1-b)Bn(Tex
)
spherical homogeneous isothermal constant
density, abundances RADEX
-gt Tmb, Dv
gt line brightness temperatures, Tb
the modelled parameter space Tkin5-500K,
N(CO)1014-5x1022cm-2 (CO) and N(CO)1012-1021
cm-2 (13CO), n(H2)103-107cm-3 grid
log(Tkin)0.05, logN(CO)0.1, logn(H2)0.5
adopted the collisional rate coeff. of
Flower(2001).
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! unknown sff
common solution use the intensity ratios (e.g.,
Johansson et al 1998, Heikkila et al. 1999,
Bolatto et al. 2005)

• sff equal for all transitions
• sff lt/ 100

additionally, restrict the range of solutions by
c2 approach
c25 95 fit, GOOD c2gt/10 60 fit, BAD!
! unknown 12CO/13CO abundance ratios
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• 12CO/13CO 12C/13C isotope ratio

• Isotopic charge exchange (Watson et al. 1976)
• 13C 12CO 12C 13CO DE ( DE/k35K, k2
  10-10 cm3s-1)
• 2. Selective isotopic photodissociation (Bally
  Langer 1982)

If 1gt2 12CO/13COexp( -DE/kTkin) x 12C/13C
Solar 89 Local ISM 68

• Galactic values
• 12CH/13CH 78/-12.7 (Cassasus et al. 2005)
• 12CO/13CO 57/-7 (Burgh et al. 2006)
• r Oph A, c Oph 125/-23, 117/-35 (Federman et
  al. 2003)
• z Oph 170 (Lambert et al. 1994)
• 77/-7 (WilsonRood 1994)
• C18O 57-74 (LangerPenzias 1993)
• CO vibrational 86-137 (Goto et al. 2003)
• CN N1-0 30-140 (Milan et al. 2005) gt201/-15
  (WouterlootBrand 1996)

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! unknown 12CO/13CO abundance ratios -
12C/13C30-75 in N159-W, from CS and HCO
(Johansson et al. 1998 - 12C/13C40-90 in
Lirs 49, from HCO, H13CO and
12CO/13CO20-40 in Lirs49, 30Dor-10, N159-W,
N159-S (Heikkila et al. 1999) -
12C/13Cgt100, based on metallicity arguments
(Lequeux et al. 1994)

• Van Dishoeck Black (1988) 12C/13C12CO/13CO
  in dark/dense clouds
• but in translucent NO!
• isotope selective photodissociation
• low-T C-isotope exchange reactions

in the MC due to the intense UV-radiation
fields, possibly 12CO/13COgt12C/13C
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!
!
!
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for 12CO/13CO5-300, and n(H2)103-105cm-3 for
a given Tkin and N(CO)
Class A sources, the SMC-B11 type, the only
other group member is
N159-S in the LMC
Class B sources, the Lirs36 type, the rest of the
sample, but Lirs49c2
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Note that most likely the local or absolute c2
minima at low isotope ratios for the Class B
sources is due to indistinguishable low and high
optical depth solutions, all these minima fall
close to the observed antena temperature ratios
of isotopic species 12CO/13CO 10. Further,
sources close to regions of vigorous star
formation, e.g., N66 and 30 Dor-10, tend to have
higher hydrogen densities and lower filling
factors, possibly indicating a higher
dissociation rate in the clouds' outer envelopes
forcing the surviving CO to the denser
regions. Also, providing that the 12CO/13CO
ratios are similar, LMC clouds have CO column
densities an order of magnitude larger than SMC
clouds. It scales directly with any possible
difference of the 12CO/13CO ratios in two
galaxies. Simulations - 1-component gas
simulations show that for Class A sources the
observed CO data are well explained only for
isotopic ratios gt50.
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- The 2-component gas models with radiatively
decoupled sum of a cold, dense component and a
warmer, lower density component, reproduce well
the observed isotope ratio plots for isotope
ratios gt50. The right panel shows Lirs 36 for a
mixture of a dominating cold component with
N(CO)2x1017 cm-2 and a warm component with
n(H2) 103 cm-3 and N(CO)5x1015 cm-2 for a fixed
isotope ratio of 100, the Tkin20K (cold) and
Tkin100K (hot gas) and for the cold gas
component n(H2) 104 cm-3 , the remaining
parameters were adjusted to resemble the observed
intensity ratios. This is obviously not a unique
solution.
observed
modelled
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Source classification (mixed our model is
unable to classify the source)
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Summary and conclusions - The derived CO column
densities are largely independent of the n(H2)
and scale with the 12CO/13CO ratio adopted. For
some clouds they are factor of 5 higher than
those previously published a discrepancy
explained in terms of the higher 12CO/13CO
ratio we used. - The surface filling factor, sff,
and kinetic temperature are strongly dependent
on the n(H2). In the SMC the upper limits of sff
are 10-20, in the LMC are a factor of a few
larger. With increasing star formation activity
the sff tends to decrease. - If similar types of
clouds are considered, CO column densities seem
to be by a factor of 10 larger in the LMC
relative to those of the SMC this discrepancy
mirrors the metallicity difference between the
two galaxies. - Defined by the c2 variations, we
have identified two classes of sources, denoted
as Class A and B. Class A objects are well
described by a simple model consisting of a
uniform, single gas component. The simulations
indicate a lower limit of the 12CO/13CO isotopic
ratio of 50.
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- The high c2 values obtained for the Class B
sources strongly indicate that the simple model
is a poor approximation to the actual conditions
of the environments. A 2-component model shows
that the observed c2 minima at 12CO/13CO ratios
lt30 are a signiture of the presence of gas
gradients and low optical depth solutions forced
by the observed 12CO/13CO brightness temperature
ratios. - Tentative results from 2-component
modelling assuming a fixed 12CO/13CO ratio of
two radiatively decoupled gas phase components of
equal surface filling factors show that the
majority of the clouds can be classified as
either of hot core type, i.e., the warmer gas
component is the denser one, or hot envelope
objects, where the warmer gas phase is more
diffuse. THE END ...
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PhD thesis research projects available 1.
Triggered star formation in Orion the IC2118
region (stars), young stars, YSOs, cores and
chemical signatures. 2. Chemistry of dense
cores and PDRs the L1219 dark cloud N-,
S-chemistry and molecular ions 3. 12C/13C ratio
in Galactic and extragalactic molecular clouds
observations and modelling collaborations with
M. Kun (1,2), D. Mardones (1), J. Eisloffel (1)
SOLD!
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THE END ...