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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+/-0.024mag

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Oblaci u oblacima – molekulski gas u Magelanovim oblacima

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

  2. 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)

  3. 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)

  4. N88 N66 30Dor-10 SMC-B1#1 N159-W Lirs 36 Lirs49 Hodge 15 N159-S

  5. X=2x1020 cm-2 K km/s H2? 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)

  6. 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). SEST1987-2003, 15m 70-356GHz

  7. observations SMC: CO(1-0), CO(2-1), 13CO(1-0), 13CO(2-1), Rubio et al. (1996) , newCO(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”

  8. 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) spherical homogeneous isothermal constant density, abundances + RADEX -> Tmb, Dv => 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).

  9. ! 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

  10. 12CO/13CO = 12C/13C isotope ratio: • 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=1-030-140 (Milan et al. 2005); >201+/-15 (Wouterloot&Brand 1996)

  11. ! 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]

  12. ! ! !

  13. 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

  14. 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 >50.

  15. - 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

  16. Source classification (“mixed” our model is unable to classify the source)

  17. 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.

  18. - 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. THE END ...

  19. 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!

  20. THE END ...

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