Download Nikolic_Silvana

Oblaci u oblacima –
molekulski gas u Magelanovim oblacima
Silvana Nikolic
Astronomska opservatorija, Beograd
image: NOAO/AURA/NSF
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] Keller&Wood (2006)
but also -1.2[Fe/H] Van den Bergh (2006)
Meixner et al. 2006
LMC –
distance mod. 18.54+/-0.018mag
mean metal. -0.34+/-0.03[Fe/H] dex Keller&Wood (2006)
but also -0.6[Fe/H] dex Van den Bergh (2006)
N88
N66
30Dor-10
SMC-B1#1
N159-W
Lirs49
Lirs 36
Hodge 15
N159-S
H2?
X=2x1020 cm-2 K km/s
X=N(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 (Strong&Mattox 1996)
ICO(1-0) => 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 dishoeck&Black 1988, Kopp et al. 2000)
CO surveys:
LMC – 1988, Cohen et al. Columbia 1.2m @115GHz HPBW 8.'8
ESO/SEST key project 1992-1995
1999, Fukui et al., NANTEN I 4m, @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
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)
Tsys=1000, 180, 150K
@ 345, 146 and 98GHz resp.
mb=0.74, 0.66, 0.33
@ 100, 147 and 345GHz resp.
HPBW= 50”, 34” and 15”
RADEX – a non-LTE excitation and radiative transfer code (Jansen et al. 1994)
4 the radaitive transfer equations the mean escape probability (MEP)
approximation.
collisions+spontaneous+stimulated radiative transitions
computes statistical equilibrium for rotational levels of IS molecules
4the internal radiation field (incl. 325 transitions, 26 levels)
In=b[Bn(TCBG)]+(1-b)Bn(Tex)
-> Tmb, Dv
spherical
homogeneous
isothermal
constant density,
abundances
+
RADEX
=> line brightness temperatures, Tb
the modelled parameter space: Tkin=5-500K,
N(CO)=1014-5x1022cm-2 (CO) and N(CO)=1012-1021 cm-2 (13CO),
n(H2)=103-107cm-3
grid: log(Tkin)=0.05, log[N(CO)]=0.1, log[n(H2)]=0.5;
adopted the collisional rate coeff. of Flower(2001).
! 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 </= 100%
additionally, restrict the range of solutions by c2 approach
c2=5 – 95% fit, GOOD
c2>/=10 – 60% fit, BAD!
! unknown [12CO/13CO] abundance ratios
-
12CO/13CO
= 12C/13C isotope ratio:
1. Isotopic charge exchange (Watson et al. 1976) :
13C+
+ 12CO
12C+
+ 13CO +DE ( DE/k=35K, k=2 10-10 cm3s-1)
2. Selective isotopic photodissociation (Bally & Langer 1982)
If 1>2:
12CO/13CO=exp(
-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 (Wilson&Rood 1994)
C18O: 57-74 (Langer&Penzias 1993)
 CO vibrational 86-137 (Goto et al. 2003)
 CN N=1-0 30-140 (Milan et al. 2005); >201+/-15 (Wouterloot&Brand 1996)
! unknown [12CO/13CO] abundance ratios
- [12C/13C]=30-75 in N159-W, from CS and HCO+ (Johansson et al. 1998
- [12C/13C]=40-90 in Lirs 49, from HCO+, H13CO+ and
[12CO/13CO]=20-40 in Lirs49, 30Dor-10, N159-W, N159-S
(Heikkila et al. 1999)
- [12C/13C]>100, based on metallicity arguments (Lequeux et al. 1994)
Van Dishoeck & Black (1988): [12C/13C]=[12CO/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/13CO]>[12C/13C]
!
!
!
for [12CO/13CO]=5-300, and n(H2)=103-105cm-3 for a given Tkin and N(CO)
Class A sources, the SMC-B1#1 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
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 Dor10, 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 >50.
- 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 >50.
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
Tkin=20K (cold) and Tkin=100K (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
Source classification (“mixed” our model is unable to classify the source)
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.
- 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 <30 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.
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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)
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