The relationship between Indian monsoon rainfall and low-pressure systems

Indian summer monsoon precipitation is significantly modulated by synoptic-scale tropical low pressure areas (LPAs), the strongest of which are known as monsoon depressions (MDs). Despite their apparent importance, previous studies attempting to constrain the fraction of monsoon precipitation for which such systems are responsible have yielded an unsatisfyingly wide range of estimates. Here, a variant of the DBSCAN algorithm is implemented to identify nontrivial, coherent rainfall structures in TRMM-3B42 precipitation data. Using theoretical considerations and an idealised model, an effective capture radius is computed to be 200 km, providing upper-bound attribution fractions of 57% (17%) for LPAs (MDs) over the monsoon core zone and 44% (12%) over all India. These results are also placed in the context of simpler attribution techniques. A climatology of these clusters suggests that the central Bay of Bengal (BoB) is the region of strongest synoptic organisation. A k-means clustering technique is used to identify four distinct partitions of LPA (and two of MD) track, and their regional contributions to monsoon precipitation are assessed. Most synoptic rainfall over India is attributable to short-lived LPAs originating at the head of the BoB, though longer-lived systems are required to bring rain to west India and east Pakistan. Secondary contributions from systems originating in the Arabian Sea and south BoB are shown to be important for west Pakistan and Sri Lanka respectively. Finally, a database of precipitating-event types is used to show that small-scale deep convection happens independently of MDs, whereas the density of larger-scale convective and stratiform events are sensitive to their presence - justifying the use of a noise-rejecting algorithm

How does star formation proceed in the circumnuclear starburst ring of NGC 6951?

Gas inflowing along stellar bars is often stalled at the location of circumnuclear rings, that form an effective reservoir for massive star formation and thus shape the central regions of galaxies. However, how exactly star formation is proceeding within these circumnuclear starburst rings is subject of debate. Two main scenarios for this process have been put forward: In the first the onset of star formation is regulated by the total amount of gas present in the ring with star forming starting once a mass threshold has reached in a `random' position within the ring like `popcorn'. In the second star formation preferentially takes place near the locations where the gas enters the ring. This scenario has been dubbed `pearls-on-a-string'. Here we combine new optical IFU data covering the full stellar bar with existing multi-wavelength data to study in detail the 580 pc radius circumnuclear starburst ring in the nearby spiral galaxy NGC 6951. Using HST archival data together with Sauron and Oasis IFU data, we derive the ages and stellar masses of star clusters as well as the total stellar content of the central region. Adding information on the molecular gas distribution, stellar and gaseous dynamics and extinction, we find that the circumnuclear ring in NGC 6951 is ~1-1.5 Gyr old and has been forming stars for most of that time. We see evidence for preferred sites of star formation within the ring, consistent with the `pearls-on-a-string' scenario, when focusing on the youngest stellar populations. Due to the ring's longevity this signature is washed out when older stellar populations are included in the analysis.Comment: accepted for publication in A&A, 15 page

ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052

We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7 observations of an extremely metal-poor dwarf starburst galaxy in the Local Universe, SBS0335-052 (12+log(O/H)~7.2). With these observations, dust is detected at 870micron (ALMA Band 7), but 87% of the flux in this band is due to free-free emission from the starburst. We have compiled a spectral energy distribution (SED) of SBS0335-052 that spans almost 6 orders of magnitude in wavelength and fit it with a spherical dust shell heated by a single-age stellar population; the best-fit model gives a dust mass of (3.8+/-0.6)x10^4 Msun. We have also constructed a SED including Herschel archival data for IZw18, another low-metallicity dwarf starburst (12+log(O/H)=7.17), and fit it with a similar model to obtain a dust mass of (3.4+/-1.0)x10^2 Msun. Compared with their atomic gas mass, the dust mass of SBS0335-052 far exceeds the prediction of a linear trend of dust-to-gas mass ratio with metallicity, while IZw18 falls far below. We use gas scaling relations to assess a putative missing gas component in both galaxies and find that the missing, possibly molecular, gas in SBS0335-052 is a factor of 6 times higher than the value inferred from the observed HI column density; in IZw18 the missing component is 4 times smaller. Ultimately, despite their similarly low metallicity, the differences in gas and dust column densities in SBS0335-052 and IZw18 suggest that metal abundance does not uniquely define star-formation processes. At some level, self-shielding and the survival of molecules may depend just as much on gas and dust column density as on metallicity. The effects of low metallicity may at least be partially compensated for by large column densities in the interstellar medium.Comment: 15 pages, 11 figures, accepted for publication in A&

The Spitzer View of Low-Metallicity Star Formation: II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus

(abridged) We present new Spitzer, UKIRT and MMT observations of the blue compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of 12+log(O/H)=8.0. This galaxy has the peculiarity of possessing an extraordinarily dense nuclear star-forming region, with a central density of ~10^6 cm^{-3}. The nuclear region of Mrk 996 is characterized by several unusual properties: a very red color J-K = 1.8, broad and narrow emission-line components, and ionizing radiation as hard as 54.9 eV, as implied by the presence of the OIV 25.89 micron line. The nucleus is located within an exponential disk with colors consistent with a single stellar population of age >1 Gyr. The infrared morphology of Mrk 996 changes with wavelength. The IRS spectrum shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission, with narrow line widths and equivalent widths that are high for the metallicity of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model requires that they originate in two distinct HII regions: a very dense HII region of radius ~580 pc with densities declining from ~10^6 at the center to a few hundreds cm^{-3} at the outer radius, where most of the optical lines arise; and a HII region with a density of ~300 cm^{-3} that is hidden in the optical but seen in the MIR. We suggest that the infrared lines arise mainly in the optically obscured HII region while they are strongly suppressed by collisional deexcitation in the optically visible one. The hard ionizing radiation needed to account for the OIV 25.89 micron line is most likely due to fast radiative shocks propagating in an interstellar medium. A hidden population of Wolf-Rayet stars of type WNE-w or a hidden AGN as sources of hard ionizing radiation are less likely possibilities.Comment: 48 pages, 13 figures, accepted for publication in the Astrophysical Journa

The effect of soil moisture perturbations on Indian Monsoon Depressions in a numerical weather prediction model

Indian monsoon depressions (MDs) are synoptic-scale cyclonic systems that propagate across peninsular India three or four times per monsoon season. They are responsible for the majority of rainfall in agrarian north India, thus constraining precipitation estimates is of high importance. Here, we use a case study from August 2014 to explore the relationship between varying soil moisture and the resulting track and structure of an incident MD using the Met Office Unified Model. We use this case study with the view to increasing understanding of the general impact of soil moisture perturbations on monsoon depressions. It is found that increasing soil moisture in the monsoon trough region results in deeper inland penetration and a more developed structure - e.g. a warmer core in the mid-troposphere and a stronger bimodal potential vorticity core in the middle/lower troposphere - with more precipitation, and a structure that in general more closely resembles that found in depressions over the ocean, indicating that soil moisture may enhance the convective mechanism that drives depressions over land. This experiment also shows that these changes are most significant when the depression is deep, and negligible when it is weakening. Increasing soil moisture in the sub-Himalayan arable zone, a region with large irrigation coverage, also caused deeper inland penetration and some feature enhancement in the upper troposphere but no significant changes were found in the track heading or lower-tropospheric structure

Halo dust detection around NGC 891

Observations of edge-on galaxies allow us to investigate the vertical extent and properties of dust, gas and stellar distributions. NGC 891 has been studied for decades and represents one of the best studied cases of an edge-on galaxy. We use deep PACS data together with IRAC, MIPS and SPIRE data to study the vertical extent of dust emission around NGC 891. We also test the presence of a more extended, thick dust component. By performing a convolution of an intrinsic vertical profile emission with each instrument PSF and comparing it with observations we derived the scaleheight of a thin and thick dust disc component. For all wavelengths considered the emission is best fit with the sum of a thin and a thick dust component. The scaleheight of both dust components shows a gradient passing from 70 $\mu$m to 250 $\mu$m. This could be due to a drop in dust heating (and thus dust temperature) with the distance from the plane, or to a sizable contribution ($\sim 15 - 80$) of an unresolved thin disc of hotter dust to the observed surface brightness at shorter wavelengths. The scaleheight of the thick dust component, using observations from 70 $\mu$m to 250 $\mu$m has been estimated to be $(1.44\pm 0.12)$ kpc, consistent with previous estimates (extinction and scattering in optical bands and MIR emission). The amount of dust mass at distances larger than $\sim 2$ kpc from the midplane represents $2 - 3.3$ % of the total galactic dust mass and the relative abundance of small grains with respect to large grains is almost halved comparing to that in the midplane. The paucity of small grains high above the midplane might indicate that dust is hit by interstellar shocks or galactic fountains and entrained together with gas. The halo dust component is likely to be embedded in an atomic / molecular gas and heated by a thick stellar disc.Comment: 15 pages, 13 figures, A&A accepte

Subtropical westerly jet influence on occurrence of western disturbances and Tibetan plateau vortices

Western disturbances (WDs) are mid-to-upper-tropospheric mesoscale vortices, which typically propagate along the subtropical westerly jet stream and bring heavy rainfall to Pakistan and northern India during boreal winter. They are dynamically similar to Tibetan Plateau vortices (TPVs), which affect southwest China during spring and summer and emanate from the Tibetan Plateau. Here, we propose that their similarity implies the existence of a more general group of upper-tropospheric vortices featuring interactions with the orography of the Hindu Kush-Himalaya-Tibetan Plateau region. Using existing track databases for WDs and TPVs derived from ERA-Interim reanalysis, we show that their respective occurrence frequencies are highly anticorrelated with each other through the seasonal cycle, yet both are strongly correlated with jet latitude. Our findings imply that the incidence of hazards due to WDs and TPVs is correlated on intra- and interannual timescales, particularly through upper-level baroclinicity

An ABC transporter containing a forkhead-associated domain interacts with a serine-threonine protein kinase and is required for growth of Mycobacterium tuberculosis in mice

Forkhead-associated (FHA) domains are modular phosphopeptide recognition motifs with a striking preference for phosphothreonine-containing epitopes. FHA domains have been best characterized in eukaryotic signaling pathways but have been identified in six proteins in Mycobacterium tuberculosis, the causative organism of tuberculosis. One of these, coded by gene Rv1747, is an ABC transporter and the only one to contain two such modules. A deletion mutant of Rv1747 is attenuated in a mouse intravenous injection model of tuberculosis where the bacterial load of the mutant is 10-fold lower than that of the wild type in both lungs and spleen. In addition, growth of the mutant in mouse bone marrow-derived macrophages and dendritic cells is significantly impaired. In contrast, growth of this mutant in vitro was indistinguishable from that of the wild type. The mutant phenotype was lost when the mutation was complemented by the wild-type allele, confirming that it was due to mutation of Rv1747. Using yeast two-hybrid analysis, we have shown that the Rv1747 protein interacts with the serine-threonine protein kinase PknF. This interaction appears to be phospho-dependent since it is abrogated in a kinase-dead mutant and by mutations in the presumed activation loop of PknF and in the first FHA domain of Rv1747. These results demonstrate that the protein coded by Rv1747 is required for normal virulent infection by M. tuberculosis in mice and, since it interacts with a serine-threonine protein kinase in a kinase-dependent manner, indicate that it forms part of an important phospho-dependent signaling pathway

Scaling relations and baryonic cycling in local star-forming galaxies: II. Gas content and star-formation efficiency

Assessments of the cold-gas reservoir in galaxies are a cornerstone for understanding star-formation processes and the role of feedback and baryonic cycling in galaxy evolution. Here we exploit a sample of 392 galaxies (dubbed MAGMA, Metallicity and Gas for Mass Assembly), presented in a recent paper, to quantify molecular and atomic gas properties across a broad range in stellar mass, Mstar, from ∼107 - 1011 M⊙. First, we find the metallicity (Z) dependence of the conversion factor for CO luminosity to molecular H2 mass αCO to be shallower than previous estimates, with αCO∝ (Z/Z⊙)-1.55. Second, molecular gas mass MH2 is found to be strongly correlated with Mstar and star-formation rate (SFR), enabling predictions of MH2 good to within ∼0.2 dex; analogous relations for atomic gas mass MHI and total gas mass Mgas are less accurate, ∼0.4 dex and ∼0.3 dex, respectively. Indeed, the behavior of atomic gas mass MHI in MAGMA scaling relations suggests that it may be a third, independent variable that encapsulates information about the circumgalactic environment and gas accretion. If Mgas is considered to depend on MHI, together with Mstar and SFR, we obtain a relation that predicts Mgas to within ∼0.05 dex. Finally, the analysis of depletion times and the scaling of MHI/Mstar and MH2/Mstar over three different mass bins suggests that the partition of gas and the regulation of star formation through gas content depends on the mass regime. Dwarf galaxies (Mstar∝ 3 × 109 M⊙) tend to be overwhelmed by (H » I) accretion, and despite short τH2 (and thus presumably high star-formation efficiency), star formation is unable to keep up with the gas supply. For galaxies in the intermediate Mstar "gas-equilibrium"bin (3 × 109 M⊙ ≲ Mstar ≲ 3 × 1010 M⊙), star formation proceeds apace with gas availability, and H I and H2 are both proportional to SFR. In the most massive "gas-poor, bimodality"regime (Mstar ≳ 3 × 1010 M⊙), H I does not apparently participate in star formation, although it generally dominates in mass over H2. Our results confirm that atomic gas plays a key role in baryonic cycling, and is a fundamental ingredient for current and future star formation, especially in dwarf galaxies

Coupling transport and biodegradation of VOCs in surface and subsurface soils.

Volatile organic chemicals present at Superfund sites preferentially partition into the soil gas and may be available for microbial degradation. A simple mass transfer model for biodegradation for volatile substrates has been developed for the aerobic decomposition of aromatic and aliphatic hydrocarbons. The mass transfer analysis calculates diffusive fluxes from soil gas through water and membrane films and into the cell. This model predicts an extreme sensitivity of potential biodegradation rates to the air-water partition coefficients of the compounds. Aromatic hydrocarbons are removed rapidly while the aliphatic hydrocarbons are much slower by orders of magnitude. Furthermore, oxygen transfer is likely to limit aromatic hydrocarbon degradation rates. The model presents results that cast doubt on the practicality of using methane or propane for the co-metabolic destruction of trichloroethylene in a gas phase bioreactor. Toluene as a primary substrate has better mass transfer characteristics to achieve more efficient trichloroethylene degradation. Hence, in sites where these contaminants coexist, bioremediation could be improved