Studies of the chemistry and physics of starless and prestellar cores

Concentrations of dense and cold gas embedded in gigantic molecular clouds, the so-called starless or prestellar cores, represent the initial stages of star formation in the interstellar medium. The dominating gas component in the cores is molecular hydrogen, but a variety of other elements such as helium, oxygen, carbon and nitrogen are also present in appreciable amounts. A notable addition to this list is deuterium which can, at the low temperatures prevalent in the cores, be treated as another element. In fact, it turns out that deuterium is increasingly important toward the very centers of the cores where the density is highest. In addition to the gas, interstellar dust is also present in the cores with a total mass of about one hundreth of the total gas mass. Chemical interaction between the gas and the dust and the relatively high gas density allow for rich chemical evolution to take place during the lifetime of a core. The chemistry affects the gas temperature because the gas cools mainly by molecular line radiation, and the gas temperature in turn determines the thermal pressure which is mainly responsible for stabilizing the core against gravitational collapse. Consequently, the chemical composition of a core influences its physical evolution. Observing the molecular line radiation emitted by the various chemical species yields information on, e.g., the kinematics and the temperature of the gas. We can also deduce molecular column densities based on observations. However, the observationally measured column densities are averages over different lines of sight and hence possible spatial variations in the number densities of the various species, and consequently in the gas temperature, can be difficult to detect. This is particularly important toward the centers of the cores where molecules are expected to freeze onto the surfaces of dust grains and hence to disappear from the gas phase. For this reason, numerical models of the chemistry are needed to correctly interpret the various observations. Indeed, studying the chemical and physical properties of starless and prestellar cores theoretically, using numerical methods, is the focus of this thesis which consists of five original journal articles. In two of the articles, we discuss the chemistry of light deuterated ions in the context of the so-called complete depletion model, in which all elements heavier than helium are frozen onto the surfaces of dust grains. In the latter of these two articles, we expand upon the complete depletion model by explicitly taking into account the chemical evolution of heavier species preceding freeze-out, and our analysis yields the first theoretical prediction of the eventual depletion of the singly-deuterated hydrogen molecule, HD. One of the articles focuses solely on the physics of starless and prestellar cores by studying their stability against gravitational collapse. For the physical core model we choose the modified Bonnor-Ebert sphere, and in this article we present for the first time a stability condition for such a sphere. We find that the stability of the modified Bonnor-Ebert sphere is similar to that of the classical isothermal Bonnor-Ebert sphere. One of the articles studies the interaction between the chemistry and physics in starless and prestellar cores. We combine chemical and radiative transfer modeling to derive estimates of the gas temperature based on simulated molecular abundances. We find that the gas temperature is expected to change by a 1-2 K during a typical core lifetime and conclude that this may have implications on the stability of the cores. Also, we find that the NH3/N2H+ abundance ratio increases at very high densities, seemingly contradicting observations. In one article, we study the chemical and physical properties of the starless core SL42. We fit observed C18O and N2H+ line profiles using simulated chemical abundances based on time-dependent and steady-state chemical models, and we find that the observed C18O abundance profile is well fitted by the steady-state model.Valtavien molekyylipilvien sisällä sijaitsevat tiheän ja kylmän kaasun tiivistymät, niin kutsutut tähdettömät ja prestellaariset ytimet, edustavat tähtien synnyn alkuvaiheita tähtienvälisessä aineessa. Ytimien kaasu koostuu enimmäkseen vedystä, mutta seassa on myös muita yleisiä alkuaineita kuten heliumia, happea, hiiltä ja typpeä. Tähän listaan voidaan lisätä myös deuterium eli raskas vety, jota voidaan ytimien alhaisista lämpötiloista johtuen käsitellä kuin omana alkuaineenaan. Osoittautuu, että deuteriumin läsnäolon merkitys kasvaa kohti ytimien kylmiä keskusalueita, joissa väliaineen tiheys on korkeimmillaan. Ytimissä on kaasun seassa tähtienvälistä pölyä, jonka kokonaismassa on noin sadasosa kaasun kokonaismassasta. Kaasu ja pöly vuorovaikuttavat keskenään kemiallisesti. Tämä vuorovaikutus muuttaa kaasun ja pölyn koostumusta ytimen elinaikana. Kemia vaikuttaa kaasun lämpötilaan, sillä kaasu jäähtyy pääasiassa molekyylien viivasäteilyn kautta. Kaasun lämpötila määrää termisen paineen, joka tasapainottaa ytimen gravitaation aiheuttamaa luhistumista vastaan. Näin ytimen kemiallinen koostumus vaikuttaa sen fysikaaliseen kehitykseen. Eri molekyylien lähettämän viivasäteilyn havaitseminen antaa tietoa esimerkiksi kaasun liikkeistä ja sen lämpötilasta. Voimme myös johtaa molekyylien määrän näkösäteellä. Kuitenkin pilven sisällä tapahtuvien paikallisten runsaus- tai lämpötilavaihtelujen havaitseminen on hankalaa. Tämä rajoitus on erityisen vakava ytimien keskustoissa, joissa molekyylien odotetaan jäätyvän pölyhiukkasten pinnalle ja näin häviävän kaasusta. Tarvitsemme siis numeerisia kemiamalleja, jotta havaintoja voidaan tulkita oikein. Tässä väitöskirjassa, joka koostuu viidestä alkuperäisestä tutkimusartikkelista, tutkin tähdettömien ja prestellaaristen ytimien kemiallisia ja fysikaalisia ominaisuuksia numeerisin menetelmin. Kahdessa artikkelissa käsittelen kevyiden deuteroituneiden ionien kemiaa "complete depletion" -mallin kontekstissa, jossa kaikkien heliumia raskaampien alkuaineiden oletetaan jäätyneen pölyhiukkasten pinnalle. Jälkimmäisessä näistä kahdesta julkaisusta laajennan "complete depletion" -mallia ottamalla eksplisiittisesti mukaan raskaiden yhdisteiden kemiallisen kehityksen ennen niiden jäätymistä pölyhiukkasten pinnoille, ja tutkimukseni johtaa ensimmäiseen teoreettiseen ennusteeseen raskaan vetymolekyylin, HD:n, vähittäisestä katoamisesta kaasussa. Yksi artikkeli keskittyy tähdettömien ja prestellaaristen ytimien tasapainoon gravitaation aiheuttamaa luhistumista vastaan. Ytimen fysikaaliseksi malliksi valitsen niin kutsutun muunnetun Bonnor-Ebert -pallon, ja esittelen ensimmäisen tasapainoehdon tällaiselle pallolle. Tutkimukseni perusteella muunnetun Bonnor-Ebert -pallon tasapaino on samanlainen kuin klassisen, isotermisen Bonnor-Ebert pallon tasapaino. Yhdessä artikkelissa tutkin tähdettömien ja prestellaaristen ytimien kemiallisen kehityksen ja fysikaalisten ominaisuuksien kytkentää. Yhdistämällä kemiallisen mallin säteilynkuljetusmalliin johdan mallinnettuihin molekyylien runsauksiin perustuvia arvioita kaasun lämpötilalle. Osoitan, että kaasun lämpötilan muutos ytimen tyypillisen eliniän aikana on noin 1-2 kelviniä ja päättelen, että tällä saattaa olla vaikutusta ytimien tasapainoon. Tutkimuksen perusteella ammoniakin (NH3) ja protonoituneen ditypen (N2H+) runsaussuhde kasvaa korkeita tiheyksiä kohden, mikä on näennäisesti ristiriidassa havaintojen kanssa. Yhdessä artikkelissa tutkitaan tähdettömän ytimen SL42 kemiallisia ja fysikaalisia ominaisuuksia. Sovitan havaittuja C18O- ja N2H+ -viivaemissioprofiileja vastaaviin simuloituihin profiileihin käyttäen sekä ajasta riippuvia että kemialliseen tasapainoon perustuvia malleja. Havaittu C18O -profiili voidaan sovittaa hyvin käyttäen yksinkertaista mallia, jossa jäätyminen ja haihtuminen pölyn pinnalta ovat tasapainossa

The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration

The deuterium fraction [N$_2$D$^+$]/[N$_2$H$^+$], may provide information about the ages of dense, cold gas structures, important to compare with dynamical models of cloud core formation and evolution. Here we introduce a complete chemical network with species containing up to three atoms, with the exception of the Oxygen chemistry, where reactions involving H$_3$O$^+$ and its deuterated forms have been added, significantly improving the consistency with comprehensive chemical networks. Deuterium chemistry and spin states of H$_2$ and H$_3^+$ isotopologues are included in this primarily gas-phase chemical model. We investigate dependence of deuterium chemistry on model parameters: density ($n_{\rm H}$), temperature, cosmic ray ionization rate, and gas-phase depletion factor of heavy elements ($f_{\rm D}$). We also explore the effects of time-dependent freeze-out of gas-phase species and dynamical evolution of density at various rates relative to free-fall collapse. For a broad range of model parameters, the timescales to reach large values of $D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$, observed in some low- and high-mass starless cores, are relatively long compared to the local free-fall timescale. These conclusions are unaffected by introducing time-dependent freeze-out and considering models with evolving density, unless the initial $f_{\rm D} \gtrsim$ 10. For fiducial model parameters, achieving $D_{\rm frac}^{\rm N_2H^+} \gtrsim 0.1$ requires collapse to be proceeding at rates at least several times slower than that of free-fall collapse, perhaps indicating a dynamically important role for magnetic fields in the support of starless cores and thus the regulation of star formation.Comment: 23 pages, 18 figures, accepted by Ap

Search for H₃⁺ isotopologues toward CRL 2136 IRS 1

Context. Deuterated interstellar molecules frequently have abundances relative to their main isotopologues much higher than the overall elemental D-to-H ratio in the cold dense interstellar medium. H₃⁺ and its isotopologues play a key role in the deuterium fractionation; however, the abundances of these isotopologues have not been measured empirically with respect to H₃⁺ to date. Aims. Our aim was to constrain the relative abundances of H₂D⁺ and D₃⁺ in the cold outer envelope of the hot core CRL 2136 IRS 1. Methods. We carried out three observations targeting H₃⁺ and its isotopologues using the spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In addition, the CO overtone band at 2.3 μm was observed by iSHELL to characterize the gas on the line of sight. Results. The H₃⁺ ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H₂D⁺ and D₃⁺ resulted in non-detections. The 3σ upper limits of N(H₂D⁺)/N(H₃⁺) and N(D₃⁺)/N(H₃⁺) are 0.24 and 0.13, respectively. The population diagram for CO is reproduced by two components of warm gas with the temperatures 58 and 530 K, assuming a local thermodynamic equilibrium (LTE) distribution of the rotational levels. Cold gas (<20 K) makes only a minor contribution to the CO molecular column toward CRL 2136 IRS 1. Conclusions. The critical conditions for deuterium fractionation in a dense cloud are low temperature and CO depletion. Given the revised cloud properties, it is no surprise that H₃⁺ isotopologues are not detected toward CRL 2136 IRS 1. The result is consistent with our current understanding of how deuterium fractionation proceeds

Multi-line observations of CH3_{3}OH, c-C3_{3}H2_{2} and HNCO towards L1544: Dissecting the core structure with chemical differentiation

Pre-stellar cores are the basic unit for the formation of stars and stellar systems. The anatomy of the physical and chemical structures of pre-stellar cores is critical for understanding the star formation process. L1544 is a prototypical pre-stellar core, which shows significant chemical differentiation surrounding the dust peak. We aim to constrain the physical conditions at the different molecular emission peaks. This study allows us to compare the abundance profiles predicted from chemical models together with the classical density structure of Bonnor-Ebert (BE) sphere. We conducted multi-transition pointed observations of CH$_{3}$OH, c-C$_{3}$H$_{2}$ and HNCO with the IRAM 30m telescope, towards the dust peak and the respective molecular peaks of L1544. With non-LTE radiative transfer calculations and a 1-dimensional model, we revisit the physical structure of L1544, and benchmark with the abundance profiles from current chemical models. We find that the HNCO, c-C$_{3}$H$_{2}$ and CH$_{3}$OH lines in L1544 are tracing progressively higher density gas, from $\sim$10$^{4}$ to several times 10$^{5}$ cm$^{-3}$. Particularly, we find that to produce the observed intensities and ratios of the CH$_{3}$OH lines, a local gas density enhancement upon the BE sphere is required. This suggests that the physical structure of an early-stage core may not necessarily follow a smooth decrease of gas density profile locally, but can be intercepted by clumpy substructures surrounding the gravitational center. Multiple transitions of molecular lines from different molecular species can provide a tomographic view of the density structure of pre-stellar cores. The local gas density enhancement deviating from the BE sphere may reflect the impact of accretion flows that appear asymmetric and are enhanced at the meeting point of large-scale cloud structures.Comment: accepted by A&A; 22 pages, 22 figures incl. appendice

Sleep problems in pain patients entering tertiary pain care : the role of pain-related anxiety, medication use, self-reported diseases, and sleep disorders

Peer reviewe

Sex-Dependent Improvement in Survival of Parkinson's Disease Patients

Background Advances in the treatment of Parkinson's disease (PD) and changes in general life expectancy may have improved survival in patients with PD. Objective The objective of this study was to investigate recent trends in PD mortality. Methods In total, 1521 patients with PD in local and national registries were followed for 11 years (2006-2016) from diagnosis until exit date or death, and the causes of death were recorded. Results The survival of men with PD improved during the follow-up period, but no change was observed in women (2-year postdiagnosis survival in men, 79.0%-86.3%, P = 0.03; 2-year postdiagnosis survival in women, 82.8%-87.5%, P = 0.42). Pneumonia was the most common immediate cause of death. Discussion The survival of men with PD has improved in Finland without a similar change in women. Because changes in treatment likely affect both sexes similarly, the results may reflect the decreasing sex gap in life expectancy. This phenomenon will likely increase the already high male-to-female prevalence ratio of PD.Peer reviewe

Search for H₃⁺ isotopologues toward CRL 2136 IRS 1

Context. Deuterated interstellar molecules frequently have abundances relative to their main isotopologues much higher than the overall elemental D-to-H ratio in the cold dense interstellar medium. H₃⁺ and its isotopologues play a key role in the deuterium fractionation; however, the abundances of these isotopologues have not been measured empirically with respect to H₃⁺ to date. Aims. Our aim was to constrain the relative abundances of H₂D⁺ and D₃⁺ in the cold outer envelope of the hot core CRL 2136 IRS 1. Methods. We carried out three observations targeting H₃⁺ and its isotopologues using the spectrographs CRIRES at the VLT, iSHELL at IRTF, and EXES on board SOFIA. In addition, the CO overtone band at 2.3 μm was observed by iSHELL to characterize the gas on the line of sight. Results. The H₃⁺ ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of H₂D⁺ and D₃⁺ resulted in non-detections. The 3σ upper limits of N(H₂D⁺)/N(H₃⁺) and N(D₃⁺)/N(H₃⁺) are 0.24 and 0.13, respectively. The population diagram for CO is reproduced by two components of warm gas with the temperatures 58 and 530 K, assuming a local thermodynamic equilibrium (LTE) distribution of the rotational levels. Cold gas (<20 K) makes only a minor contribution to the CO molecular column toward CRL 2136 IRS 1. Conclusions. The critical conditions for deuterium fractionation in a dense cloud are low temperature and CO depletion. Given the revised cloud properties, it is no surprise that H₃⁺ isotopologues are not detected toward CRL 2136 IRS 1. The result is consistent with our current understanding of how deuterium fractionation proceeds

Nuclear spin ratios of deuterated ammonia in prestellar cores. LAsMA observations of H-MM1 and Oph D

We determine the ortho/para ratios of NH2D and NHD2 in two dense, starless cores, where their formation is supposed to be dominated by gas-phase reactions, which, in turn, is predicted to result in deviations from the statistical spin ratios. The Large APEX sub-Millimeter Array (LAsMA) multibeam receiver of the Atacama Pathfinder EXperiment (APEX) telescope was used to observe the prestellar cores H-MM1 and Oph D in Ophiuchus in the ground-state lines of ortho and para NH2D and NHD2. The fractional abundances of these molecules were derived employing 3D radiative transfer modelling, using different assumptions about the abundance profiles as functions of density. We also ran gas-grain chemistry models with different scenarios concerning proton or deuteron exchanges and chemical desorption from grains to find out if one of these models can reproduce the observed spin ratios. The observationally deduced ortho/para ratios of NH2D and NHD2 are in both cores within 10% of their statistical values 3 and 2, respectively, and taking 3-sigma limits, deviations from these of about 20% are allowed. Of the chemistry models tested here, the model that assumes proton hop (as opposed to full scrambling) in reactions contributing to ammonia formation, and a constant efficiency of chemical desorption, comes nearest to the observed abundances and spin ratios. The nuclear spin ratios derived here are in contrast with spin-state chemistry models that assume full scrambling in proton donation and hydrogen abstraction reactions leading to deuterated ammonia. The efficiency of chemical desorption influences strongly the predicted abundances of NH3, NH2D, and NHD2, but has a lesser effect on their ortho/para ratios. For these the proton exchange scenario in the gas is decisive. We suggest that this is because of rapid re-processing of ammonia and related cations by gas-phase ion-molecule reactions.Comment: accepted for publication in Astronomy & Astrophysic