WISE J044232.92+322734.9: A star-forming galaxy at redshift 1.1 seen through a Galactic dust clump?

Physically unassociated background or foreground objects seen towards submillimetre sources are potential contaminants of both the studies of young stellar objects embedded in Galactic dust clumps and multiwavelength counterparts of submillimetre galaxies (SMGs). We employed the near-infrared and mid-infrared data from the Wide-field Infrared Survey Explorer (WISE) and the submillimetre data from the Planck satellite, and uncovered a source, namely WISE J044232.92+322734.9, whose WISE infrared colours suggest that it is a star-forming galaxy (SFG), and which is seen in projection towards the Planck-detected dust clump PGCC G169.20-8.96. We used the MAGPHYS+photo-$z$ spectral energy distribution code to derive the photometric redshift and physical properties of J044232.92. The redshift was derived to be $z_{\rm phot}=1.132^{+0.280}_{-0.165}$, while, for example, the stellar mass, IR (8-1 000 $\mu$m) luminosity, and star formation rate were derived to be $M_{\star}=4.6^{+4.7}_{-2.5}\times10^{11}$ M$_{\odot}$, $L_{\rm IR}=2.8^{+5.7}_{-1.5}\times10^{12}$ L$_{\odot}$, and ${\rm SFR}=191^{+580}_{-146}$ ${\rm M}_{\odot}$ yr$^{-1}$. The derived value of $L_{\rm IR}$ suggests that J044232.92 could be an ultraluminous infrared galaxy, and we found that it is consistent with a main sequence SFG at a redshift of 1.132. Moreover, the estimated physical properties of J044232.92 are comparable to those of SMGs. Further observations, in particular high-resolution (sub-)millimetre and radio continuum imaging, are needed to better constrain the redshift and physical properties of J044232.92 and to see if the source really is a galaxy seen through a Galactic dust clump, in particular an SMG population member at $z\sim1.1$.Comment: 7 pages, 4 figures, 3 tables, accepted for publication in A&A, abstract abridged for arXi

APEX observations of ortho-H2_2D+^+ towards dense cores in the Orion B9 filament

We used the APEX telescope to observe the 372 GHz o-H$_2$D$^+(J_{K_a,\,K_c}=1_{1,\,0}-1_{1,\,1})$ line towards three prestellar cores and three protostellar cores in Orion B9. We also employed our previous APEX observations of C$^{17}$O, C$^{18}$O, N$_2$H$^+$, and N$_2$D$^+$ line emission, and 870 $\mu$m dust continuum emission towards the target sources. The o-H$_2$D$^+$ line was detected in all three prestellar cores, but in only one of the protostellar cores. The corresponding o-H$_2$D$^+$ abundances were derived to be $\sim(12-30)\times10^{-11}$ and $6\times10^{-11}$. Two additional spectral lines, DCO$^+(5-4)$ and N$_2$H$^+(4-3)$, were detected in the observed frequency bands with high detection rates of $100\%$ and $83\%$, respectively. The Orion B9 cores were found to be consistent with the relationship between the o-H$_2$D$^+$ abundance and gas temperature obeyed by other low-mass dense cores. The o-H$_2$D$^+$ abundance was found to decrease as the core evolves. The o-H$_2$D$^+$ abundances in the Orion B9 cores are in line with those found in other low-mass cores and larger than derived for high-mass star-forming regions. The higher o-H$_2$D$^+$ abundance in prestellar cores compared to that in cores hosting protostars is to be expected from chemical reactions where higher concentrations of gas-phase CO and elevated gas temperature accelerate the destruction of H$_2$D$^+$. The validity of using the [o-H$_2$D$^+$]/[N$_2$D$^+$] ratio as an evolutionary indicator, which has been proposed for massive clumps, remains to be determined when applied to these target cores. Overall, as located in a dynamic environment of Orion B, the Orion B9 filament provides an interesting target system to investigate the deuterium-based chemistry, and further observations of species like para-H$_2$D$^+$ and D$_2$H$^+$ would be of particular interest.Comment: 15 pages (incl. an appendix), 8 figures, 7 tables, accepted for publication in A&A, abstract abridged for arXi

What did the seahorse swallow? APEX 170 GHz observations of the chemical conditions in the Seahorse infrared dark cloud

We used the APEX telescope to observe spectral lines occurring at about 170 GHz frequency towards 14 positions along the full extent of the filamentary Seahorse infrared dark cloud. Six spectral line transitions were detected ($\geq3\sigma$) altogether, namely, SO$(N_J=4_4-3_3)$, H$^{13}$CN$(J=2-1)$, H$^{13}$CO$^+(J=2-1)$, SiO$(J=4-3)$, HN$^{13}$C$(J=2-1)$, and C$_2$H$(N=2-1)$. While SO, H$^{13}$CO$^+$, and HN$^{13}$C were detected in every source, the detection rates for C$_2$H and H$^{13}$CN were 92.9% and 85.7%, respectively. Only one source (SMM 3) showed detectable SiO emission (7.1% detection rate). Three clumps (SMM 5, 6, and 7) showed the SO, H$^{13}$CN, H$^{13}$CO$^+$, HN$^{13}$C, and C$_2$H lines in absorption. We found three positive correlations among the derived molecular abundances, of which those between C$_2$H and HN$^{13}$C and HN$^{13}$C and H$^{13}$CO$^+$ are the most significant (correlation coefficient $r\simeq0.9$). The statistically most significant evolutionary trends we uncovered are the drops in the C$_2$H abundance and in the $[{\rm HN^{13}C}]/[{\rm H^{13}CN}]$ ratio as the clump evolves from an IR dark stage to an IR bright stage and then to an HII region. The correlations we found between the different molecular abundances can be understood as arising from the gas-phase electron (ionisation degree) and atomic carbon abundances. The [C$_2$H] evolutionary indicator we found is in agreement with previous studies, and can be explained by the conversion of C$_2$H to other species (e.g. CO) when the clump temperature rises, especially after the ignition of a hot molecular core in the clump. The decrease of $[{\rm HN^{13}C}]/[{\rm H^{13}CN}]$ as the clump evolves is also likely to reflect the increase in the clump temperature, which leads to an enhanced formation of HCN and its $^{13}$C isotopologue.Comment: 23 pages (incl. an appendix), 13 figures, 7 tables, accepted for publication in A&A, abstract abridged for arXi

Dense cores in the Seahorse infrared dark cloud: physical properties from modified blackbody fits to the far-infrared-submillimetre spectral energy distributions

We used data from WISE, IRAS, and Herschel in conjuction with our previous observations with SABOCA and LABOCA, and constructed the far-IR to submillimetre spectral energy distributions (SEDs) of dense cores in the filamentary Seahorse infrared dark cloud (IRDC) G304.74+01.32. For the 12 analysed cores, which include two IR dark cores (no WISE counterpart), nine IR bright cores, and one HII region, the mean dust temperature of the cold (warm) component, the mass, luminosity, H$_2$ number density, and surface density were derived to be $13.3\pm1.4$ K ($47.0\pm5.0$ K), $113\pm29$ M$_{\odot}$, $192\pm94$ L$_{\odot}$, $(4.3\pm1.2)\times10^5$ cm$^{-3}$, and $0.77\pm0.19$ g cm$^{-3}$, respectively. The HII region IRAS 13039-6108a was found to be the most luminous source in our sample ($(1.1\pm0.4)\times10^3$ L$_{\odot}$). All the cores were found to be gravitationally bound (i.e. the virial parameter $\alpha_{\rm vir}<2$). Seven out of 12 of the analysed cores (58%) were found to lie above the mass-radius thresholds of high-mass star formation proposed in the literature. The surface densities of $\Sigma>0.4$ g cm$^{-3}$ derived for these seven cores also exceed the corresponding threshold for high-mass star formation. Five of the analysed cores (42%) show evidence of fragmentation into two components in the SABOCA 350 $\mu$m image. In addition to the HII region source IRAS 13039-6108a, some of the other cores in Seahorse also appear to be capable of giving birth to high-mass stars. The dense core population in the Seahorse IRDC has comparable average properties to the cores in the well-studied Snake IRDC G11.11-0.12. The Seahorse core fragmentation mechanisms appear to be heterogenous, including cases of both thermal and non-thermal Jeans instability. High-resolution follow-up studies are required to address the fragmented cores' genuine potential of forming high-mass stars.Comment: 16 pages (incl. two appendices), 9 figures, 6 tables, accepted for publication in A&A, abstract abridged for arXi

Molecular line and continuum studies of the early stages of star formation

New stars form in dense interstellar clouds of gas and dust called molecular clouds. The actual sites where the process of star formation takes place are the dense clumps and cores deeply embedded in molecular clouds. The details of the star formation process are complex and not completely understood. Thus, determining the physical and chemical properties of molecular cloud cores is necessary for a better understanding of how stars are formed. Some of the main features of the origin of low-mass stars, like the Sun, are already relatively well-known, though many details of the process are still under debate. The mechanism through which high-mass stars form, on the other hand, is poorly understood. Although it is likely that the formation of high-mass stars shares many properties similar to those of low-mass stars, the very first steps of the evolutionary sequence are unclear. Observational studies of star formation are carried out particularly at infrared, submillimetre, millimetre, and radio wavelengths. Much of our knowledge about the early stages of star formation in our Milky Way galaxy is obtained through molecular spectral line and dust continuum observations. The continuum emission of cold dust is one of the best tracers of the column density of molecular hydrogen, the main constituent of molecular clouds. Consequently, dust continuum observations provide a powerful tool to map large portions across molecular clouds, and to identify the dense star-forming sites within them. Molecular line observations, on the other hand, provide information on the gas kinematics and temperature. Together, these two observational tools provide an efficient way to study the dense interstellar gas and the associated dust that form new stars. The properties of highly obscured young stars can be further examined through radio continuum observations at centimetre wavelengths. For example, radio continuum emission carries useful information on conditions in the protostar+disk interaction region where protostellar jets are launched. In this PhD thesis, we study the physical and chemical properties of dense clumps and cores in both low- and high-mass star-forming regions. The sources are mainly studied in a statistical sense, but also in more detail. In this way, we are able to examine the general characteristics of the early stages of star formation, cloud properties on large scales (such as fragmentation), and some of the initial conditions of the collapse process that leads to the formation of a star. The studies presented in this thesis are mainly based on molecular line and dust continuum observations. These are combined with archival observations at infrared wavelengths in order to study the protostellar content of the cloud cores. In addition, centimetre radio continuum emission from young stellar objects (YSOs; i.e., protostars and pre-main sequence stars) is studied in this thesis to determine their evolutionary stages. The main results of this thesis are as follows: i) filamentary and sheet-like molecular cloud structures, such as infrared dark clouds (IRDCs), are likely to be caused by supersonic turbulence but their fragmentation at the scale of cores could be due to gravo-thermal instability; ii) the core evolution in the Orion B9 star-forming region appears to be dynamic and the role played by slow ambipolar diffusion in the formation and collapse of the cores may not be significant; iii) the study of the R CrA star-forming region suggests that the centimetre radio emission properties of a YSO are likely to change with its evolutionary stage; iv) the IRDC G304.74+01.32 contains candidate high-mass starless cores which may represent the very first steps of high-mass star and star cluster formation; v) SiO outflow signatures are seen in several high-mass star-forming regions which suggest that high-mass stars form in a similar way as their low-mass counterparts, i.e., via disk accretion. The results presented in this thesis provide constraints on the initial conditions and early stages of both low- and high-mass star formation. In particular, this thesis presents several observational results on the early stages of clustered star formation, which is the dominant mode of star formation in our Galaxy.Tähdet syntyvät tiheissä tähtienvälisissä kaasu- ja pölypilvissä joita kutsutaan molekyylipilviksi. Varsinainen tähtien syntyprosessi tapahtuu molekyylipilven tiheimmissä osissa, niin sanotuissa klumpeissa ja pilviytimissä. Prosessin yksityiskohdat ovat monimutkaisia eikä niitä vielä täysin ymmärretä. Molekyylipilviytimien fysikaalisten ja kemiallisten ominaisuuksien määrittäminen on välttämätöntä jotta ymmärtäisimme paremmin kuinka tähdet syntyvät. Pienimassaisten tähtien, kuten meidän Auringon, syntyvaiheiden pääpiirteet tunnetaan suhteellisen hyvin vaikka prosessin monet yksityiskohdat ovatkin vielä kiistanalaisia. Toisaalta mekanismit jotka johtavat suurimassaisten tähtien syntyyn ovat hämäränpeitossa. Vaikka onkin todennäköistä, että suurimassaisten tähtien synty muistuttaa monelta osin pienimassaisten tähtien syntyä, ensimmäisiä kehitysvaiheita suurimassaisten tähtien synnyssä ei tunneta. Tähtien syntyä tutkitaan erityisesti infrapuna-, (ali)millimetri-, ja radioaallonpituuksilla. Tietomme Linnunradan tähtien synnyn varhaisvaiheista perustuu suurelta osin molekyylien spektriviiva- ja pölyn kontinuumihavaintoihin. Kylmän tähtienvälisen pölyn kontinuumiemissio on yksi parhaista tavoista määrittää molekyylipilvien yleisimmän molekyylin, vedyn, pylvästiheys. Tämän johdosta pölyn kontinuumihavainnot tarjoavat tehokkaan menetelmän molekyylipilvien kartoittamiseen ja tiheiden tähtien syntyalueiden etsimiseen. Toisaalta molekyyliviivahavainnot tarjoavat tietoa kaasun kinematiikasta ja lämpötilasta. Yhdessä yo. havaintomenetelmät muodostavat tehokkaan tavan tutkia tiheää tähtienvälistä kaasua ja pölyä jotka toimivat uusien tähtien rakennusaineena. Pilviytimien uumenissa olevien nuorten tähtien ominaisuuksia voidaan edelleen tutkia senttimetrialueen radiokontinuumihavaintojen avulla. Radiokontinuumiemissio tarjoaa hyödyllistä tietoa esimerkiksi prototähden ja sitä ympäröivän kiekon välisestä vuorovaikutusalueesta, josta esim. protostellaaristen suihkujen uskotaan saavan alkunsa. Tässä väitöskirjatyössä tutkitaan sekä pieni- että suurimassaisten tähtien syntyalueilla olevien klumppien ja ytimien fysikaalisia ja kemiallisia ominaisuuksia. Kohteita tutkitaan lähinnä tilastollisessa mielessä, mutta myös yksityiskohtaisesti. Tämä mahdollistaa tähtien synnyn yleisten ominaisuuksien tarkastelemisen, emopilvien suuren skaalan ominaisuuksien (kuten fragmentoitumisen) tutkimisen, sekä tähtien syntyprosessiin liittyvien alkuolosuhteiden määrittämisen. Väitöskirjassa esitetyt tutkimukset perustuvat pääasiassa molekyyliviiva- ja pölyn kontinuumihavaintoihin. Työssä käytetään myös infrapuna-alueen arkistohavaintoja, joka mahdollistaa pilviytimissä mahdollisesti olevien prototähtien tutkimisen. Lisäksi väitöskirjassa tutkitaan nuorten tähtien (prototähdet ja esi-pääsarjan tähdet) senttimetrialueen radiokontinuumiemissiota, joka auttaa kohteiden kehitysvaiheen määrittämistä. Väitöskirjatyön päätulokset ovat seuraavat: i) filamentaariset ja "liuskamaiset" molekyylipilvirakenteet, kuten infrapunapimeät sumut (engl. infrared dark clouds, IRDCs), ovat todennäköisesti seurausta supersoonisesta tähtienvälisestä turbulenssista, mutta pilvien fragmentoituminen ytimien skaalassa on mahdollisesti seurausta niin sanotusta gravitaatioinstabilisuudesta; ii) pilviytimien kehitys Orion B9-tähtien syntyalueessa vaikuttaa olevan dynaaminen ja hitaan ambipolaarisen diffuusion rooli ytimien muodostumisessa ja romahtamisessa ei luultavasti ole tärkeä; iii) R CrA-tähtien syntyalueen tutkimus viittaa siihen, että nuorten tähtien senttimetrialueen radioemissio-ominaisuudet muuttuvat kehitysvaiheen myötä; iv) infrapunapimeässä sumussa G304.74+01.32 on luultavasti suurimassaisia tähdettömiä ytimiä, jotka voivat edustaa ensimmäisiä vaiheita suurimassaisten tähtien ja tähtijoukkojen synnyssä; v) useissa suurimassaisten tähtien syntyalueissa nähdään merkkejä ulosvirtauksista SiO-spektriviivoissa, joka viittaa siihen, että suurimassaiset tähdet syntyvät samaan tapaan kuin pienimassaiset tähdet eli keräämällä ympäröivää ainetta kertymäkiekon välityksellä. Tässä väitöskirjassa esitetyt tulokset ovat hyödyksi sekä pieni- että suurimassaisten tähtien synnyn alkuolosuhteiden ja varhaisten kehitysvaiheiden ymmärtämiseksi. Erityisesti väitöskirjassa esitetään useita havaintotuloksia joukoittain syntyvien tähtien varhaisvaiheista. Tämä tieto on tarpeellista koska suurin osa Linnunratamme uusista tähdistä syntyy joukoissa

SABOCA 350-micron and LABOCA 870-micron dust continuum imaging of IRAS 05399-0121: mapping the dust properties of a pre- and protostellar core system

We present a 350 micron APEX/SABOCA map of IRAS 05399-0121/SMM 1, which is a dense double-core system in Orion B9. We combined these data with our previous LABOCA 870-micron data. The spatial resolution of the new SABOCA image, ~3400 AU, is about 2.6 times better than provided by LABOCA. We also make use of Spitzer infrared observations to characterise the star-formation activity in the source. The source is filamentary and remains a double-core system on the 3400 AU scale probed here, where the projected separation between IRAS 05399 and SMM 1 is 0.14 pc. The broadband spectral energy distribution of IRAS 05399 suggests that it is near the Stage 0/I borderline. A visual inspection of the Spitzer/IRAC images provides hints of a quadrupolar-like jet morphology around IRAS 05399, supporting the possibility that it is a binary system. The temperature map reveals warm spots towards IRAS 05399 and the southeastern tip of the source. These features are likely to be imprints of protostellar or shock heating, while external heating could be provided by the nearby high-mass star-forming region NGC 2024. A simple analysis suggests that the density profile at the position of SMM 1 has the form ~r^-(2.3_{-0.9}^{+2.2}). The source splitting into two subcores along the long axis can be explained by cylindrical Jeans-type fragmentation but the steepness of the density profile is shallower than what is expected for an isothermal cylinder. The difference between the evolutionary stages of IRAS 05399 (protostellar) and SMM 1 (starless) suggests that the former has experienced a phase of rapid mass accretion, supported by the very long outflow it drives. The protostellar jet from IRAS 05399 might have influenced the nearby core SMM 1.Comment: A&A, in press; 14 pages, 7 figures, 3 tables; very minor language corrections+revised arXiv abstrac

Mapping the prestellar core Ophiuchus D (L1696A) in ammonia

The gas kinetic temperature in the centres of starless, high-density cores is predicted to fall as low as 5-6 K. The aim of this study was to determine the kinetic temperature distribution in the low-mass prestellar core Oph D where previous observations suggest a very low central temperature. The densest part of the Oph D core was mapped in the NH3(1,1) and (2,2) inversion lines using the Very Large Array (VLA). The physical quantities were derived from the observed spectra by fitting the hyperfine structure of the lines, and subsequently the temperature distribution of Oph D was calculated using the standard rotational temperature techniques. A physical model of the cores was constructed, and the simulated spectra after radiative transfer calculations with a 3D Monte Carlo code were compared with the observed spectra. Temperature, density, and ammonia abundance of the core model were tuned until a satisfactory match with the observation was obtained. The high resolution of the interferometric data reveals that the southern part of Oph D comprises of two small cores. The gas kinetic temperatures, as derived from ammonia towards the centres of the southern and northern core are 7.4 and 8.9 K, respectively. The observed masses of the cores are only 0.2 M_Sun. Their potential collapse could lead to formation of brown dwarfs or low-mass stars.Comment: Accepted for publication in A&A; 10 pages, 9 figure

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