91 research outputs found
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 sisÀllÀ sijaitsevat tiheÀn ja kylmÀn kaasun tiivistymÀt, niin kutsutut tÀhdettömÀt ja prestellaariset ytimet, edustavat tÀhtien synnyn alkuvaiheita tÀhtienvÀlisessÀ aineessa. Ytimien kaasu koostuu enimmÀkseen vedystÀ, mutta seassa on myös muita yleisiÀ alkuaineita kuten heliumia, happea, hiiltÀ ja typpeÀ. TÀhÀn listaan voidaan lisÀtÀ myös deuterium eli raskas vety, jota voidaan ytimien alhaisista lÀmpötiloista johtuen kÀsitellÀ kuin omana alkuaineenaan. Osoittautuu, ettÀ deuteriumin lÀsnÀolon merkitys kasvaa kohti ytimien kylmiÀ keskusalueita, joissa vÀliaineen tiheys on korkeimmillaan.
YtimissÀ on kaasun seassa tÀhtienvÀlistÀ pölyÀ, jonka kokonaismassa on noin sadasosa kaasun kokonaismassasta. Kaasu ja pöly vuorovaikuttavat keskenÀÀn kemiallisesti. TÀmÀ vuorovaikutus muuttaa kaasun ja pölyn koostumusta ytimen elinaikana. Kemia vaikuttaa kaasun lÀmpötilaan, sillÀ kaasu jÀÀhtyy pÀÀasiassa molekyylien viivasÀteilyn kautta. Kaasun lÀmpötila mÀÀrÀÀ termisen paineen, joka tasapainottaa ytimen gravitaation aiheuttamaa luhistumista vastaan. NÀin ytimen kemiallinen koostumus vaikuttaa sen fysikaaliseen kehitykseen.
Eri molekyylien lÀhettÀmÀn viivasÀteilyn havaitseminen antaa tietoa esimerkiksi kaasun liikkeistÀ ja sen lÀmpötilasta. Voimme myös johtaa molekyylien mÀÀrÀn nÀkösÀteellÀ. Kuitenkin pilven sisÀllÀ tapahtuvien paikallisten runsaus- tai lÀmpötilavaihtelujen havaitseminen on hankalaa. TÀmÀ rajoitus on erityisen vakava ytimien keskustoissa, joissa molekyylien odotetaan jÀÀtyvÀn pölyhiukkasten pinnalle ja nÀin hÀviÀvÀn kaasusta. Tarvitsemme siis numeerisia kemiamalleja, jotta havaintoja voidaan tulkita oikein. TÀssÀ vÀitöskirjassa, joka koostuu viidestÀ alkuperÀisestÀ tutkimusartikkelista, tutkin tÀhdettömien ja prestellaaristen ytimien kemiallisia ja fysikaalisia ominaisuuksia numeerisin menetelmin.
Kahdessa artikkelissa kÀsittelen kevyiden deuteroituneiden ionien kemiaa "complete depletion" -mallin kontekstissa, jossa kaikkien heliumia raskaampien alkuaineiden oletetaan jÀÀtyneen pölyhiukkasten pinnalle. JÀlkimmÀisessÀ nÀistÀ kahdesta julkaisusta laajennan "complete depletion" -mallia ottamalla eksplisiittisesti mukaan raskaiden yhdisteiden kemiallisen kehityksen ennen niiden jÀÀtymistÀ pölyhiukkasten pinnoille, ja tutkimukseni johtaa ensimmÀiseen teoreettiseen ennusteeseen raskaan vetymolekyylin, HD:n, vÀhittÀisestÀ katoamisesta kaasussa.
Yksi artikkeli keskittyy tÀhdettömien ja prestellaaristen ytimien tasapainoon gravitaation aiheuttamaa luhistumista vastaan. Ytimen fysikaaliseksi malliksi valitsen niin kutsutun muunnetun Bonnor-Ebert -pallon, ja esittelen ensimmÀisen tasapainoehdon tÀllaiselle pallolle. Tutkimukseni perusteella muunnetun Bonnor-Ebert -pallon tasapaino on samanlainen kuin klassisen, isotermisen Bonnor-Ebert pallon tasapaino.
YhdessÀ artikkelissa tutkin tÀhdettömien ja prestellaaristen ytimien kemiallisen kehityksen ja fysikaalisten ominaisuuksien kytkentÀÀ. YhdistÀmÀllÀ kemiallisen mallin sÀteilynkuljetusmalliin johdan mallinnettuihin molekyylien runsauksiin perustuvia arvioita kaasun lÀmpötilalle. Osoitan, ettÀ kaasun lÀmpötilan muutos ytimen tyypillisen eliniÀn aikana on noin 1-2 kelviniÀ ja pÀÀttelen, ettÀ tÀllÀ saattaa olla vaikutusta ytimien tasapainoon. Tutkimuksen perusteella ammoniakin (NH3) ja protonoituneen ditypen (N2H+) runsaussuhde kasvaa korkeita tiheyksiÀ kohden, mikÀ on nÀennÀisesti ristiriidassa havaintojen kanssa.
YhdessÀ artikkelissa tutkitaan tÀhdettömÀn ytimen SL42 kemiallisia ja fysikaalisia ominaisuuksia. Sovitan havaittuja C18O- ja N2H+ -viivaemissioprofiileja vastaaviin simuloituihin profiileihin kÀyttÀen sekÀ ajasta riippuvia ettÀ kemialliseen tasapainoon perustuvia malleja. Havaittu C18O -profiili voidaan sovittaa hyvin kÀyttÀen yksinkertaista mallia, jossa jÀÀtyminen ja haihtuminen pölyn pinnalta ovat tasapainossa
The Deuterium Fractionation Timescale in Dense Cloud Cores: A Parameter Space Exploration
The deuterium fraction [ND]/[NH], 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 HO and its
deuterated forms have been added, significantly improving the consistency with
comprehensive chemical networks. Deuterium chemistry and spin states of H
and H isotopologues are included in this primarily gas-phase chemical
model. We investigate dependence of deuterium chemistry on model parameters:
density (), temperature, cosmic ray ionization rate, and gas-phase
depletion factor of heavy elements (). 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 , 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 10. For fiducial
model parameters, achieving 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 Hââș 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. Hââș 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 Hââș to date.
Aims. Our aim was to constrain the relative abundances of HâDâș and Dââș in the cold outer envelope of the hot core CRL 2136 IRS 1.
Methods. We carried out three observations targeting Hââș 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 ÎŒm was observed by iSHELL to characterize the gas on the line of sight.
Results. The Hââș ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of HâDâș and Dââș resulted in non-detections. The 3Ï upper limits of N(HâDâș)/N(Hââș) and N(Dââș)/N(Hââș) 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 Hââș 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 CHOH, c-CH 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 CHOH, c-CH 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-CH
and CHOH lines in L1544 are tracing progressively higher density gas,
from 10 to several times 10 cm. Particularly, we find
that to produce the observed intensities and ratios of the CHOH 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 Hââș 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. Hââș 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 Hââș to date.
Aims. Our aim was to constrain the relative abundances of HâDâș and Dââș in the cold outer envelope of the hot core CRL 2136 IRS 1.
Methods. We carried out three observations targeting Hââș 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 ÎŒm was observed by iSHELL to characterize the gas on the line of sight.
Results. The Hââș ion was detected toward CRL 2136 IRS 1 as in previous observations. Spectroscopy of lines of HâDâș and Dââș resulted in non-detections. The 3Ï upper limits of N(HâDâș)/N(Hââș) and N(Dââș)/N(Hââș) 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 Hââș 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
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