Chemical modeling of the L1498 and L1517B prestellar cores: CO and HCO+ depletion

Prestellar cores exhibit a strong chemical differentiation, which is mainly caused by the freeze-out of molecules onto the grain surfaces. Understanding this chemical structure is important, because molecular lines are often used as probes to constrain the core physical properties. Here we present new observations and analysis of the C18O (1-0) and H13CO+ (1-0) line emission in the L1498 and L1517B prestellar cores, located in the Taurus-Auriga molecular complex. We model these observations with a detailed chemistry network coupled to a radiative transfer code. Our model successfully reproduces the observed C18O (1-0) emission for a chemical age of a few 10^5 years. On the other hand, the observed H13CO+ (1-0) is reproduced only if cosmic-ray desorption by secondary photons is included, and if the grains have grown to a bigger size than average ISM grains in the core interior. This grain growth is consistent with the infrared scattered light ("coreshine") detected in these two objects, and is found to increase the CO abundance in the core interior by about a factor four. According to our model, CO is depleted by about 2-3 orders of magnitude in the core center.Comment: Accepted for publication in A&

VEAP: a visualisation engine and analyzer for preSS#

Computer science courses have been shown to have a low rate of student retention. There are many possible reasons for this, and our research group have had considerable success in pinpointing the factors that influence outcome when learning to program. The earlier we are able to make these predictions, the earlier a teacher can intervene and provide help to an at-risk student, before they fail and/or drop out. PreSS (Predict Student Success) is a semi-automated machine learning system developed between 2002 and 2006 that can predict the performance of students on an introductory programming module with 80% accuracy, after minimal programming exposure. Between 2013 and 2015, a fully automated web-based system was developed, known as PreSS#, that replicates the original system but provides: a streamlined user interface; an easy acquisition process; automatic modeling; and reporting. Currently, the reporting component of PreSS# outputs a value that indicates if the student is a "weak" or "strong" programmer, along with a measure of confidence in the prediction. This paper will discuss the development of VEAP: a Visualisation Engine and Analyser for PreSS#. This software provides a comprehensive data visualisation and user interface, that will allow teachers to view data gathered and processed about institutions, classes and individual students, and provides access to further user-defined analysis, to allow a teacher to view how an intervention could influence a student's predicted outcome

Integrating cavity based gas cells: a multibeam compensation scheme for pathlength variation

We present a four beam ratiometric setup for an integrating sphere based gas cell, which can correct for changes in pathlength due to sphere wall contamination. This allows for the gas absorption coefficient to be determined continuously without needing to recalibrate the setup. We demonstrate the technique experimentally, measuring methane gas at 1651nm. For example, contamination covering 1.2% of the sphere wall resulted in an uncompensated error in gas absorption coefficient of ≈41%. With the ratiometric scheme, this error was reduced to ≈2%. Potential limitations of the technique, due to subsequent deviations from mathematical assumptions are discussed, including severe sphere window contamination

Exploring the Origins of Earth's Nitrogen: Astronomical Observations of Nitrogen-bearing Organics in Protostellar Environments

It is not known whether the original carriers of Earth's nitrogen were molecular ices or refractory dust. To investigate this question, we have used data and results of Herschel observations towards two protostellar sources: the high-mass hot core of Orion KL, and the low-mass protostar IRAS 16293-2422. Towards Orion KL, our analysis of the molecular inventory of Crockett et al. (2014) indicates that HCN is the organic molecule that contains by far the most nitrogen, carrying $74_{-9}^{+5}\%$ of nitrogen-in-organics. Following this evidence, we explore HCN towards IRAS 16293-2422, which we consider a solar analog. Towards IRAS 16293-2422, we have reduced and analyzed Herschel spectra of HCN, and fit these observations against "jump" abundance models of IRAS 16293-2422's protostellar envelope. We find an inner-envelope HCN abundance $X_{\textrm{in}} = 5.9\pm0.7 \times 10^{-8}$ and an outer-envelope HCN abundance $X_{\textrm{out}} = 1.3 \pm 0.1 \times 10^{-9}$. We also find the sublimation temperature of HCN to be $T_{\textrm{jump}} = 71 \pm 3$~K; this measured $T_{\textrm{jump}}$ enables us to predict an HCN binding energy $E_{\textrm{B}}/k = 3840 \pm 140$~K. Based on a comparison of the HCN/H2O ratio in these protostars to N/H2O ratios in comets, we find that HCN (and, by extension, other organics) in these protostars is incapable of providing the total bulk N/H2O in comets. We suggest that refractory dust, not molecular ices, was the bulk provider of nitrogen to comets. However, interstellar dust is not known to have 15N enrichment, while high 15N enrichment is seen in both nitrogen-bearing ices and in cometary nitrogen. This may indicate that these 15N-enriched ices were an important contributor to the nitrogen in planetesimals and likely to the Earth.Comment: Accepted to ApJ; 21 pages, 4 figure

Controlling Ozone and Fine Particulates: Cost Benefit Analysis with Meteorological Variability

In this paper, we develop an integrated cost-benefit analysis framework for ozone and fine particulate control, accounting for variability and uncertainty. The framework includes air quality simulation, sensitivity analysis, stochastic multi-objective air quality management, and stochastic cost-benefit analysis. This paper has two major contributions. The first is the development of stochastic source-receptor (S-R) coefficient matrices for ozone and fine particulate matter using an advanced air quality simulation model (URM-1ATM) and an efficient sensitivity algorithm (DDM-3D). The second is a demonstration of this framework for alternative ozone and PM2.5 reduction policies. Alternative objectives of the stochastic air quality management model include optimization of the net social benefits and maximization of the reliability of satisfying certain air quality goals. We also examine the effect of accounting for distributional concerns.ambient air, ozone, particulate matter, risk management, public policy, cost-benefit analysis, variability and uncertainty, stochastic simulation, stochastic multi-objective programming, decisionmaking, National Ambient Air Quality Standards

Source-Receptor Relationships for Ozone and Fine Particulates in the Eastern United States

A key question in developing effective mitigation strategies for ozone and particulate matter is identifying which source regions contribute to concentrations in receptor regions. Using a direct approach with a regional, multiscale three-dimensional model, we derive multiple source-receptor matrices (S-Rs) to show inter- and intrastate impacts of emissions on both ozone and PM2.5 over the eastern United States. Our results show that local (in-state) emissions generally account for about 23% of both local ozone concentrations and PM2.5 concentrations, while neighboring states contribute much of the rest. The relative impact of each state on others varies dramatically between episodes. In reducing fine particulate concentrations, we find that reducing SO2 emissions can be 10 times as effective as reducing NOx emissions. SO2 reductions can lead to some increase in nitrates, but this is relatively small. NOx reductions, however, lead to both ozone reductions and some reduction in nitrate and sulfate particulate matter.source-receptor, ozone, particulate matter, sensitivity analysis, air quality simulation, National Ambient Air Quality Standards

Atomic jet from SMM1 (FIRS1) in Serpens uncovers non-coeval binary companion

We report on the detection of an atomic jet associated with the protostellar source SMM1 (FIRS1) in Serpens. The jet is revealed in [FeII] and [NeII] line maps observed with Spitzer/IRS, and further confirmed in HiRes IRAC and MIPS images. It is traced very close to SMM1 and peaks at ~5 arcsec" from the source at a position angle of $\sim 125 degrees. In contrast, molecular hydrogen emission becomes prominent at distances > 5" from the protostar and extends at a position angle of 160 degrees. The morphological differences suggest that the atomic emission arises from a companion source, lying in the foreground of the envelope surrounding the embedded protostar SMM1. In addition the molecular and atomic Spitzer maps disentangle the large scale CO (3-2) emission observed in the region into two distinct bipolar outflows, giving further support to a proto-binary source setup. Analysis at the peaks of the [FeII] jet show that emission arises from warm and dense gas (T ~1000 K, n(electron) 10^5 - 10^6 cm^-3). The mass flux of the jet derived independently for the [FeII] and [NeII] lines is 10^7 M(sun)/yr, pointing to a more evolved Class~I/II protostar as the driving source. All existing evidence converge to the conclusion that SMM1 is a non-coeval proto-binary source.Comment: 10 pages, 7 figures, 1 table. Accepted for publication in Astronomy \& Astrophysic

A non-equilibrium ortho-to-para ratio of water in the Orion PDR

The ortho-to-para ratio (OPR) of H$_2$O is thought to be sensitive to the temperature of water formation. The OPR of H$_2$O is thus useful to study the formation mechanism of water. We investigate the OPR of water in the Orion PDR (Photon-dominated region), at the Orion Bar and Orion S positions, using data from {\it Herschel}/HIFI. We detect the ground-state lines of ortho- and para-H$_2$$^{18}$O in the Orion Bar and Orion S and we estimate the column densities using LTE and non-LTE methods. Based on our calculations, the ortho-to-para ratio (OPR) in the Orion Bar is 0.1 $-$ 0.5, which is unexpectedly low given the gas temperature of $\sim$ 85 K, and also lower than the values measured for other interstellar clouds and protoplanetary disks. Toward Orion S, our OPR estimate is below 2. This low OPR at 2 positions in the Orion PDR is inconsistent with gas phase formation and with thermal evaporation from dust grains, but it may be explained by photodesorption

C2_2H observations toward the Orion Bar

C$_2$H is one of the first radicals to be detected in the interstellar medium. Its higher rotational transitions have recently become available with the Herschel Space Observatory. We aim to constrain the physical parameters of the C$_2$H emitting gas toward the Orion Bar. We analyse the C$_2$H line intensities measured toward the Orion Bar CO$^+$ Peak and Herschel/HIFI maps of C$_2$H, CH, and HCO$^+$, and a NANTEN map of [CI]. We interpret the observed C$_2$H emission using radiative transfer and PDR models. Five rotational transitions of C$_2$H have been detected in the HIFI frequency range toward the CO$^+$ peak. A single component rotational diagram gives a rotation temperature of ~64 K and a beam-averaged C$_2$H column density of 4$\times$10$^{13}$ cm$^{-2}$. The measured transitions cannot be explained by any single parameter model. According to a non-LTE model, most of the C$_2$H column density produces the lower-$N$ C$_2$H transitions and traces a warm ($T_{\rm{kin}}$ ~ 100-150 K) and dense ($n$(H$_2$)~10$^5$-10$^6$ cm$^{-3}$) gas. A small fraction of the C$_2$H column density is required to reproduce the intensity of the highest-$N$ transitions ($N$=9-8 and N=10-9) originating from a high density ($n$(H$_2$)~5$\times$10$^6$ cm$^{-3}$) hot ($T_{\rm{kin}}$ ~ 400 K) gas. The total beam-averaged C$_2$H column density in the model is 10$^{14}$ cm$^{-2}$. Both the non-LTE radiative transfer model and a simple PDR model representing the Orion Bar with a plane-parallel slab of gas and dust suggest, that C$_2$H cannot be described by a single pressure component, unlike the reactive ion CH$^+$, which was previously analysed toward the Orion Bar CO$^+$ peak. The physical parameters traced by the higher rotational transitions ($N$=6-5,...,10-9) of C$_2$H may be consistent with the edges of dense clumps exposed to UV radiation near the ionization front of the Orion Bar.Comment: Proposed for acceptance in A&A, abstract abridge