arXiv subjects

Lars E. Kristensen

Publications and source records attributed to Lars E. Kristensen.

41 records · Page 3Linked to original sources

arXiv2011

Complex molecules toward low-mass protostars: the Serpens core

Gas-phase complex organic molecules are commonly detected toward high-mass protostellar hot cores. Detections toward low-mass protostars and outflows are comparatively rare, and a larger sample is key to investigate how the chemistry responds to its environment. Guided by the prediction that complex organic molecules form in CH3OH-rich ices and thermally or non-thermally evaporate with CH3OH, we have identified three sight-lines in the Serpens core - SMM1, SMM4 and SMM4-W - which are likely to be rich in complex organics. Using the IRAM 30m telescope, narrow lines (FWHM of 1-2 km s-1) of CH3CHO and CH3OCH3 are detected toward all sources, HCOOCH3 toward SMM1 and SMM4-W, and C2H5OH not at all. Beam-averaged abundances of individual complex organics range between 0.6 and 10% with respect to CH3OH when the CH3OH rotational temperature is applied. The summed complex organic abundances also vary by an order of magnitude, with the richest chemistry toward the most luminous protostar SMM1. The range of abundances compare well with other beam-averaged observations of low-mass sources. Complex organic abundances are of the same order of magnitude toward low-mass protostars and high-mass hot cores, but HCOOCH3 is relatively more important toward low-mass protostars. This is consistent with a sequential ice photochemistry, dominated by CHO-containing products at low temperatures and early times.

astro-ph.GA↗

arXiv2011

Distribution and excitation of thermal methanol in 6.7 GHz maser bearing star-forming regions; I. The nearby source Cepheus A

The aim of this study is to investigate the distribution and excitation of the methanol gas around Cep A and to describe the physical conditions of the region. In addition the large scale abundance distribution is determined in order to understand the morphology and kinematics of star forming regions in which methanol masers occur. The spatial distribution of the methanol is studied by mapping line emission, as well as the column density and excitation temperature, which are estimated using rotation diagrams. For a limited number of positions the parameters are checked with non-LTE models. Furthermore, the distribution of the methanol abundance is derived in comparison with archival dust continuum maps. Methanol is detected over a 0.3x0.15 pc area centred on the Cep A HW2 source, showing an outflow signature. Most of the gas can be characterized by a moderately warm rotation temperature (30-60K). At the central position two velocity components are detected with different excitation characteristics, the first related to the large-scale outflow. The second component, uniquely detected at the central location, is probably associated with the maser emission on much smaller scales of 2". Detailed analysis reveals that the highest densities and temperatures occur for these inner components. In the inner region the dust and gas are shown to have different physical parameters. Abundances of methanol in the range 10E-9 - 10E-7 are inferred, with the abundance peaking at the maser position. The geometry of the large-scale methanol is in accordance with previous determinations of the Cep A geometry, in particular those from methanol masers. The dynamical and chemical time-scales are consistent with the methanol originating from a single driving source associated with the HW2 object and the masers in its equatorial region.

astro-ph.SR↗

arXiv2011

Energetic processes revealed by spectrally resolved high-J CO lines in low-mass star-forming regions with Herschel-HIFI

Herschel-HIFI observations of high-J lines (up to J_u=10) of 12CO, 13CO and C18O are presented toward three deeply embedded low-mass protostars in NGC1333. The observations show several energetic components including shocked and quiescent gas. Radiative transfer models are used to quantify the C18O envelope abundance which require a jump in the abundance at an evaporation temperature, T_ev ~25 K, providing new direct evidence of a CO ice evaporation zone around protostars. The abundance in the outermost part of the envelope, X_0, is within the canonical value of 2x10^-4; however the inner abundance, X_in, is found around a factor of 3-5 lower than X_0.

astro-ph.SR↗

arXiv2009

Laboratory evidence for the non-detection of excited nascent H2 in dark clouds

There has always been a great deal of interest in the formation of H2 as well as in the binding energy released upon its formation on the surface of dust grains. The present work aims at collecting experimental evidence for how the bond energy budget of H2 is distributed between the reaction site and the internal energy of the molecule. So far, the non-detection of excited nascent H2 in dense quiescent clouds could be a sign that either predictions of emission line intensities are not correct or the de-excitation of the newly formed molecules proceeds rapidly on the grain surface itself. In this letter we present experimental evidence that interstellar molecular hydrogen is formed and then rapidly de-excited on the surface of porous water ice mantles. In addition, although we detect ro-vibrationally excited nascent molecules desorbing from a bare non-porous (compact) water ice film, we demonstrate that the amount of excited nascent hydrogen molecules is significantly reduced no matter the morphology of the water ice substrate at 10 K (both on non-porous and on porous water ice) in a regime of high molecular coverage as is the case in dark molecular clouds.

physics.space-ph↗

arXiv2009

Spitzer mapping of molecular hydrogen pure rotational lines in NGC 1333: A detailed study of feedback in star formation

We present mid-infrared spectral maps of the NGC 1333 star forming region, obtained with the the Infrared Spectrometer on board the Spitzer Space Telescope. Eight pure H2 rotational lines, from S (0) to S (7), are detected and mapped. The H2 emission appears to be associated with the warm gas shocked by the multiple outflows present in the region. A comparison between the observed intensities and the predictions of detailed shock models indicates that the emission arises in both slow (12 - 24 km/s) and fast (36 - 53 km/s) C-type shocks with an initial ortho-to-para ratio of ~ 1. The present H2 ortho-to-para ratio exhibits a large degree of spatial variations. In the post-shocked gas, it is usually about 2, i.e. close to the equilibrium value (~ 3). However, around at least two outflows, we observe a region with a much lower (~ 0.5) ortho-to-para ratio. This region probably corresponds to gas which has been heated-up recently by the passage of a shock front, but whose ortho-to-para has not reached equilibrium yet. This, together with the low initial ortho-to-para ratio needed to reproduce the observed emission, provide strong evidence that H2 is mostly in para form in cold molecular clouds. The H2 lines are found to contribute to 25 - 50% of the total outflow luminosity, and thus can be used to ascertain the importance of star formation feedback on the natal cloud. From these lines, we determine the outflow mass loss rate and, indirectly, the stellar infall rate, the outflow momentum and the kinetic energy injected into the cloud over the embedded phase. The latter is found to exceed the binding energy of individual cores, suggesting that outflows could be the main mechanism for core disruption.

astro-ph.SR↗