Oblaci u oblacima molekulski gas u Magelanovim oblacima

Oblaci u oblacima – molekulski gas u Magelanovim oblacima Silvana Nikolic Astronomska opservatorija, Beograd image: NOAO/AURA/NSF

S 3 MC mosaic for the Spitzer IRAC 8. 0, 4. 5 and 3. 6 m (RGB), Bolatto et al. 2007 SMC – distance mod. 18. 93+/-0. 024 mag 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. 018 mag mean metal. -0. 34+/-0. 03[Fe/H] dex Keller&Wood (2006) but also -0. 6[Fe/H] dex Van den Bergh (2006)

N 88 N 66 30 Dor-10 SMC-B 1#1 N 159 -W Lirs 49 Lirs 36 Hodge 15 N 159 -S

H 2 ? X=N(H 2)/I CO(1 -0) X=2 x 1020 cm-2 K km/s • correlation CO-Av; extrapolation N(H)/Av diffuse ISM (Savage et al. 1977) • excitation analysis 13 CO, and the 12 CO/13 CO 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(H 2)=10 -4 (Dickman 1978) but diffuse & translucent ~4 x 10 -7, ~9 x 10 -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. 2 m @115 GHz HPBW 8. '8 ESO/SEST key project 1992 -1995 1999, Fukui et al. , NANTEN I 4 m, @115 GHz HPBW 2. '6 SMC - 1991, Rubio et al. Columbia 1. 2 m 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, 15 m 70 -356 GHz

observations SMC: CO(1 -0), CO(2 -1), 13 CO(1 -0), 13 CO(2 -1), Rubio et al. (1996) , new CO(3 -2) LMC: -II- Johansson et al. (1998) Tsys=1000, 180, 150 K @ 345, 146 and 98 GHz resp. mb=0. 74, 0. 66, 0. 33 @ 100, 147 and 345 GHz 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 4 the 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 -500 K, N(CO)=1014 -5 x 1022 cm-2 (CO) and N(CO)=1012 -1021 cm-2 (13 CO), n(H 2)=103 -107 cm-3 grid: log(Tkin)=0. 05, log[N(CO)]=0. 1, log[n(H 2)]=0. 5; adopted the collisional rate coeff. of Flower(2001).

! unknown sff v v 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 c 2 approach c 2=5 – 95% fit, GOOD c 2>/=10 – 60% fit, BAD! ! unknown [12 CO/13 CO] abundance ratios

- 12 CO/13 CO = 12 C/13 C isotope ratio: 1. Isotopic charge exchange (Watson et al. 1976) : 2. 13 C+ + 12 CO 12 C+ + 13 CO +DE ( DE/k=35 K, k=2 10 -10 cm 3 s-1 3. 2. Selective isotopic photodissociation (Bally & Langer 1982) If 1>2: 12 CO/13 CO=exp( -DE/k. Tkin) x 12 C/13 C Solar: 89 Local ISM ~68 Galactic values: § 12 CH+/13 CH+ 78+/-12. 7 (Cassasus et al. 2005) § 12 CO/13 CO 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) §C 18 O: 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 [12 CO/13 CO] abundance ratios - [12 C/13 C]=30 -75 in N 159 -W, from CS and HCO+ (Johansson et al. 1998 - [12 C/13 C]=40 -90 in Lirs 49, from HCO+, H 13 CO+ and [12 CO/13 CO]=20 -40 in Lirs 49, 30 Dor-10, N 159 -W, N 159 -S (Heikkila et al. 1999) - [12 C/13 C]>100, based on metallicity arguments (Lequeux et al. 1994) Van Dishoeck & Black (1988): [12 C/13 C]=[12 CO/13 CO] 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 [12 CO/13 CO]>[12 C/13 C]

for [12 CO/13 CO]=5 -300, and n(H 2)=103 -105 cm-3 for a given Tkin and N(CO) Class A sources, the SMC-B 1#1 type, the only other group member is N 159 -S in the LMC Class B sources, the Lirs 36 type, the rest of the sample, but Lirs 49 c 2

Note that most likely the local or absolute c 2 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 12 CO/13 CO ~ 10. Further, sources close to regions of vigorous star formation, e. g. , N 66 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 [12 CO/13 CO] 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 [12 CO/13 CO] 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)=2 x 1017 cm-2 and a warm component with n(H 2)= 103 cm-3 and N(CO)=5 x 1015 cm-2 for a fixed isotope ratio of 100, the Tkin=20 K (cold) and Tkin=100 K (hot gas) and for the cold gas component n(H 2)= 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(H 2) and scale with the [12 CO/13 CO] ratio adopted. For some clouds they are ~ factor of 5 higher than those previously published – a discrepancy explained in terms of the higher [12 CO/13 CO] ratio we used. - The surface filling factor, sff, and kinetic temperature are strongly dependent on the n(H 2). 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 c 2 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 12 CO/13 CO isotopic ratio of ~50.

- The high c 2 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 c 2 minima at [12 CO/13 CO] ratios <30 are a signiture of the presence of gas gradients and low optical depth solutions forced by the observed 12 CO/13 CO brightness temperature ratios. - Tentative results from 2 -component modelling assuming a fixed [12 CO/13 CO] 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. . .

Ph. D thesis research projects available: 1. Triggered star formation in Orion – the IC 2118 region: (stars), young stars, YSOs, cores and chemical signatures. 2. Chemistry of dense cores and PDRs – the L 1219 dark cloud: N-, S-chemistry and molecular ions 3. 12 C/13 C 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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