IUCAA Preprints

Permanent URI for this communityhttp://localhost:4000/handle/11007/149

Browse

Search Results

Now showing 1 - 10 of 17

Suggestions for an interstellar C7H2 Search
(2011-07-06) Chandra, Suresh; Musrif, P. G.; Dharmkare, Ram M.
Laboratory detection of the ring-chain molecule c-C7H2 has been reported by McCarthy et al. [McCarthy, M.C., Travers, M.J., Gottlieb, C.A., Thaddeus, P., 1997. A&A 483, L139]. Two ring-chain molecules c-C3H2 and c-C5H2 of this series have already been detected in the cosmic objects. We suggest that the c-C7H2 may be identiﬁed in cool cosmic objects through its transitions 414–505,515–606,616–707 and 717–808 at 23.241, 21.105, 18.953 and 16.787 GHz, respectively, in absorption against the CMB. Since, in absence of the availability of collisional rates, we have used scaled values for them, we have checked the sensitivity of the results on the collisional rates, by enhancing the rates for the transitions with Dka = 0 by a factor of 10. Though the transitions are not found sensitive to the collisional rates, our results still may be treated as qualitative in nature. These absorption lines may play an important role for identiﬁcation of c-C H in cool cosmic objects.
VLT-UVES survey for molecular hydrogen in high-redshift damped Lyman-alpha systems: Physical conditions in the neutral gas
(2005-06-01) Srianand, R.; Petitjean, Patrick; Ledoux, C´edric; et al.
We study the physical conditions in damped Lyman-α systems (DLAs), using a sample of 33 systems toward 26 QSOs acquired for a recently completed survey of H2 by Ledoux et al. (2003). We use the column densities of H2 in diﬀerent rotational levels, together with those of C i, C i ∗, C i ∗∗, C ii ∗ and singly ionized atomic species to discuss the kinetic temperature, the density of hydrogen and the electronic density in the gas together with the ambient UV radiation ﬁeld. Detailed comparisons are made between the observed properties in DLAs, the interstellar medium (ISM) of the Galaxy, the large and small Magellanic clouds (LMC and SMC).The mean kinetic temperature of the gas corresponding to DLA subcomponents in which H2 absorption line is detected, derived from the ortho-to-para ratio (153±78 K), is higher than that measured in the ISM (77±17 K) and the Magellanic clouds (82±21 K). Typical pressure in these components (corresponding to T = 100−300 K and nH = 10−200 cm−3), measured using C i ﬁne-structure excitation, are higher than what is measured along ISM sightlines. This is consistent with the corresponding higher values for N(H2,J=2)/N(H2,J=0) seen in DLAs. From the column densities of the high-J rotational levels, we derive that the typical radiation ﬁeld in the H2 bearing components is of the order of or slightly higher than the mean UV ﬁeld in the Galactic ISM. Determination of electron density in the gas with H2 and C i show the ionization rate is similar to that of a cold neutral medium (CNM) in a moderate radiation ﬁeld. This, together with the fact that we see H2 in 13-20% of the DLAs, can be used to conclude that DLAs at z > 1.9 could contribute as much as 50% star formation rate density seen in Lyman break galaxies (LBGs).C ii ∗ absorption line is detected in all the components where H2 absorption line is seen. The excitation of C ii in these systems is consistent with the physical parameters derived from the excitation of H2 and C i. We detect C ii ∗ in about 50% of the DLAs and therefore in a considerable fraction of DLAs that do not show H2. In part of the later systems, physical conditions could be similar to that in the CNM gas of the Galaxy. However, the absence of C i absorption line and the presence of Al iii absorption lines with a proﬁle similar to the proﬁles of singly ionized species suggest an appreciable contribution from warm (WNM) and/or partially ionized gas. The absence of H2, for the level of metallicity and dust depletion seen in these systems, are consistent with low densities (i.e nH 6 1 cm−3) for a radiation ﬁeld similar to the mean Galactic UV ﬁeld.
Physical conditions in the ISM towards HD185418
(2005-06-01) Shaw, Gargi; Srianand, R.
We have developed a complete model of the hydrogen molecule as part of the spectral simulation code Cloudy. Our goal is to apply this to spectra of high-redshift star-forming regions where H2 absorption is seen, but where few other details are known, to understand its implication for star formation. The microphysics of H2 is intricate, and it is important to validate these numerical simulations in better-understood environments. This paper studies a well-defined line-of-sight through the Galactic interstellar medium (ISM) as a test of the microphysics and methods we use. We present a self-consistent calculation of the observed absorption-line spectrum to derive the physical conditions in the ISM towards HD185418, a ine-of-sight with many observables. We deduce density, temperature, local radiation field, cosmic ray ionization rate, chemical composition and compare these conclusions with conditions deduced from analytical calculations. We find a higher density, similar abundances, and require a cosmic ray flux enhanced over the Galactic background value, consistent with enhancements redicted by MHD simulations.
Properties of dust in early-type galaxies
(2006-11-12) Patil, M. K.; Pandey, S. K.; Sahu, D. K.; et al.
We report optical extinction properties of dust for a sample of 26 early-type galaxies based on the analysis of their multicolour CCD observations. The wavelength dependence of dust extinction for these galaxies is determined and the extinction curves are found to run parallel to the Galactic extinction curve, which implies that the properties of dust in the extragalactic environment are quite similar to those of the Milky Way. For the sample galaxies, value of the parameter RV, the ratio of total extinction in V band to selective extinction in B & V bands, lies in the range 2.03 - 3.46 with an average of 3.02, compared to its canonical value of 3.1 for the Milky Way. A dependence of RV on dust morphology of the host galaxy is also noticed in the sense that galaxies with a well deﬁned dust lane show tendency to have smaller RV values compared to the galaxies with disturbed dust morphology. The dust content of these galaxies estimated using total optical extinction is found to lie in the range 104 to 106 M⊙, an order of magnitude smaller than those derived from IRAS ﬂux densities, indicating that a signiﬁcant fraction of dust intermixed with stars remains undetected by the optical method. We examine the relationship between dust mass derived from IRAS ﬂux and the X-ray luminosity of the host galaxies.The issue of the origin of dust in early-type galaxies is also discussed.
On the enhanced cosmic-ray ionization rate in the diffuse cloud towards Zeta Persei
(2007-10-27) Shaw, Gargi; Stancil, P. C.; Ferland, G. J.; et al.
The spatial distribution of the cosmic-ray flux is important in understanding the Interstellar Medium (ISM) of the Galaxy. This distribution can be analyzed by studying different molecular species along different sight lines whose abundances are sensitive to the cosmic-ray ionization rate. Recently several groups have reported an enhanced cosmic-ray ionization rate (ζ=χCRζstandard) in diffuse clouds compared to the standard value, ζstandard (=2.5x10-17 s-1), measured toward dense molecular clouds. In an earlier work we reported an enhancement χCR =20 towards HD185418. McCall et al. have reported χCR =48 towards ζ Persei based on the observed abundance of H3+ while Le Petit et al. found χCR ≈ 10 to be consistent with their models for this same sight line. Here we revisit ζ Persei and perform a detailed calculation using a self-consistent treatment of the hydrogen chemistry, grain physics, energy and ionization balance, and excitation physics. We show that the value of χCR deduced from the H3 + column density, N(H3 +), in the diffuse region of the sightline depends strongly on the properties of the grains because they remove free electrons and change the hydrogen chemistry. The observations are largely consistent with χCR ≈ 40, with several diagnostics indicating higher values. This underscores the importance of a full treatment of grain physics in studies of interstellar chemistry.
Interstellar Extinction by Spheroidal Dust Grains
(2005-06-01) Gupta, Ranjan
Observations of interstellar extinction and polarization indicate that the interstellar medium consists of aligned non-spherical dust grains which show variation in the interstellar extinction curve for wavelengths ranging from NIR to UV. To model the extinction and polarization, one cannot use the conventional Mie theory which assumes the grains as solid spheres. We have used a T-matrix based method for computing the extinction eﬃcien- cies of spheroidal silicate and graphite grains of diﬀerent shapes (axial ratios) and sizes and used these eﬃciencies to evaluate the interstellar extinction curve in the wavelength range 3.4 − 0.1 m. A best ﬁt linear combination of silicate and graphite grains of not very large axial ratio, ﬁts the observed extinction curve reasonably well. We calculate the volume extinction factor Vc, which is an important parameter from the point of view of the cosmic abundance, for the spheroidal grain models that reproduce the interstellar extinction curve. We ﬁnd that the shape of the grains do not aﬀect the volume extinction factor. Finally we have also studied the extinction and linear polarization eﬃciencies for aligned spheroids. The results show that the shape of grains aﬀects the linear polarization eﬃciencies considerably for various orientation angles of the spheroids.
Hot Halos around High Redshift Protogalaxies: Observations of O VI and N V Absorption in Damped Lyman Alpha systems
(2007-01-15) Fox, Andrew J.; Petitjean, Patrick; Ledoux, C´edric; et al.
Aims. We present a study of the highly ionized gas (plasma) associated with damped Lyman-α (DLA) systems at z=2.1–3.1. Methods. We search for Ovi absorption and corresponding Si iv, Civ, and Nv in a Very Large Telescope/Ultraviolet-Visible Echelle Spectrograph (VLT/UVES) sample of 35 DLA systems with data covering Ovi at S/N>10. We then use optical depth proﬁle comparisons and ionization modelling to investigate the properties, phase structure, and origin of the plasma. Results. We report twelve DLAs (nine intervening and three at <5 000 kms −1 from the QSO redshift) with detections of Ovi absorption. There are no clear Ovi non-detections, so the incidence of Ovi in DLAs is between 34% (12/35) and 100%. Among these 12 DLAs, Civ and Si iv are seen whenever data is available, and Nv is detected in 3 cases. Analysis of the line widths together with photoionization modelling suggests that two phases of DLA plasma exist: a hot, collisionally ionized phase (seen in broad Ovi components), and a warm, photoionized phase (seen just in narrow Civ and Si iv components). The presence of inﬂows and/or outﬂows is indicated by individual Ovi and Civ components displaced from the neutral gas (either blueshifted or redshifted) by up to 400 kms −1 . We ﬁnd tentative evidence (98% conﬁdence) for correlations between the DLA metallicity (measured in the neutral gas) and high-ion column density, and between the DLA metallicity and high-ion line width, as would be expected if supernova-driven galactic outﬂows rather than accretion produced the high ions. Using conservative ionization corrections, we ﬁnd lower limits to the total hydrogen column densities in the hot (Ovi-bearing) and warm (Civ-bearing) phases in the range logNHot H ii > 19.5 to > 21.1, and logNWarm H ii > 19.4 to > 20.9. On average, the hot and warm phases thus contain > ∼40% and > ∼20% of the baryonic mass of the neutral phase in DLAs, respectively. Conclusions. If the temperature in the Ovi phase is ≈ 106 K and so fOvi = Ovi/O ≪ 0.2, the plasma can make a signiﬁcant contribution to the metal budget at high redshift. Additional searches for Ovi in Lyman Limit Systems (QSO absorbers with 17.0 < NHi < 20.3) will be necessary to determine the total quantity of baryons and metals hidden in hot halos at z ≈ 2.
Galactic dynamos supported by magnetic helicity fluxes
(2007-03-08) Sur, Sharanya; Shukurov, A.; Subramanian, Kandaswamy
We present a simple semi-analytical model of nonlinear, mean-ﬁeld galactic dynamos and use it to study the effects of various magnetic helicity ﬂuxes. The dynamo equations are reduced using the ‘no-z’ approximation to a nonlinear system of ordinary differential equations in time; we demonstrate that the model reproduces accurately earlier results, including those where nonlinear behaviour is driven by a magnetic helicity ﬂux. We discuss the implications and interplay of two types of magnetic helicity ﬂux, one produced by advection (e.g., due to the galactic fountain or wind) and the other, arising fromanisotropy of turbulence as suggested by Vishniac & Cho (2001). We argue that the latter is signiﬁcant if the galactic differential rotation is strong enough: in ourmodel, forRω . −10 in terms of the corresponding turbulent magnetic Reynolds number. We conﬁrm that the intensity of gas outﬂow from the galactic disc optimal for the dynamo action is close to that expected for normal spiral galaxies. The steady-state strength of the large-scale magnetic ﬁeld supported by the helicity advection is still weaker than that corresponding to equipartition with the turbulent energy. However, the Vishniac-Cho helicity ﬂux can boost magnetic ﬁeld further to achieve energy equipartition with turbulence. For stronger outﬂows that may occur in starburst galaxies, the Vishniac-Cho ﬂux can be essential for the dynamo action. However, this mechanism requires a large-scale magnetic ﬁeld of at least≃ 1 Gto be launched, so that it has to be preceded by a conventional dynamo assisted by the advection of magnetic helicity by the fountain or wind.
Critical properties of spherically symmetric accretion in a fractal medium
(2007-07-18) Roy, Nirupam; Ray, Arnab K.
Spherically symmetric transonic accretion of a fractal medium has been studied in both the stationary and the dynamic regimes. The stationary transonic solution is greatly sensitive to inﬁnitesimal deviations in the outer boundary condition, but the ﬂow becomes transonic and stable, when its evolution is followed through time. The evolution towards transonicity is more pronounced for a fractal medium than what is it for a continuum. The dynamic approach also shows that there is a remarkable closeness between an equation of motion for a perturbation in the ﬂow, and the metric of an analogue acoustic black hole. The stationary inﬂow solutions of a fractal medium are as much stable under the inﬂuence of linearised perturbations, as they are for the ﬂuid continuum.
Metallicity as a criterion to select H2 bearing damped lyman-alpha systems
(2006-07-17) Petitjean, Patrick; Ledoux, C.; Noterdaeme, P.; et al.

IUCAA Preprints

Browse

Filters

Settings

Sort By

Results per page

Search Results