11 research outputs found

    Multi-Wavelength Observations of the High-Mass Star Forming Complexes W33 and DR 21

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    High-mass stars play a key role in shaping the Universe. Stars with masses above 8 M_sun have a huge impact on the energy budget of galaxies, through their stellar winds, expanding HII regions, outflows, and supernova explosions. Although insight into the formation of stars is gained from low-mass star forming regions, the knowledge about high-mass star formation is still incomplete and has to be increased through new observations and theories. Observationally, different stages of star formation are distinguished. A cold molecular cloud collapses and fragments into smaller entities ("clumps" and "cores"). The cores contract further and slowly start to warm up their center where a protostar is formed. The protostar grows through accretion of material while at a certain point, hydrogen burning sets in. The evolving protostar starts to heat up its birth cloud, changing the chemistry in different layers around the star (hot core phase). An HII region is formed once the radiation of the protostar is energetic enough to ionize the surrounding material. With time, the radiation of the protostar destroys the birth cloud and the star becomes observable at optical wavelengths. In the course of this dissertation, the formation of high-mass stars was studied along the described evolutionary sequence on the basis of multi-wavelength observations of the high-mass star forming complexes W33 and DR 21. In W33, molecular clouds in several stages of star formation are detected, from quiescent cold clouds to highly active HII regions. Two radial velocity components with a difference of ~20 km/s were detected towards different parts of the complex. For a long time, this raised the question if W33 is a physically connected star forming complex or if the star forming regions with different radial velocities are located at different distances along the line-of-sight. Due to this peculiar velocity structure, the distance to W33 was not well known. As part of the dissertation, the trigonometric parallax distance of W33 was determined with Very Long Baseline Interferometry observations of water masers in the complex, yielding a distance of 2.4 kpc. Since the star forming regions with the different radial velocity components are located at similar distances, we conclude that W33 is physically connected. Furthermore, these observations yield the proper motions of the water masers from which we inferred the internal motions of the star forming regions and the motions of these regions within the W33 complex. Since the clouds in the W33 complex are physically linked and are in different stages of star formation, we conducted a chemical study of these clouds with single dish and interferometer observations at submillimeter wavelengths to gather information about the chemical composition on different scales along the evolutionary sequence. On larger scales, the number of detected molecules and their complexity increases from the prestellar phase to the HII region phase. On smaller scales, the clouds in the hot core phase show the highest chemical complexity and diversity. The observed molecules, some of them quite complex, were generated on the dust grains and then released into the gas phase as primary molecules or produced in the gas phase by the evaporated molecules as secondary molecules. In the HII region phase, almost no complex molecules are detected anymore and the spectrum resembles the spectra of the clouds in the protostellar phase before the excitation of a hot core. The complex molecules are either destroyed by photo-dissociation or their emission is not compact enough to be detected by the interferometer. With interferometer observations at radio wavelengths, we looked for hypercompact and ultracompact HII regions in the W33 Main cloud. We detected an ultracompact HII region and inferred the spectral type of the dominating star which ionizes the surrounding material. The ultracompact HII region has an arc-shape similar to cometary HII regions. Furthermore, water masers and a Class I methanol maser are detected in the W33 Main cloud. While the water masers are probably associated with an outflow in W33 Main, the methanol maser is located offset from any dust or molecular line emission and it is not clear what powers it. DR 21 contains two cometary HII regions, whose sizes classify them as ultracompact and compact HII regions. To study the velocity field of the two HII regions, we analyzed archival radio recombination line observations of DR 21. We detected two velocity components in the tails of both HII regions which indicate the presence of stellar winds. Stellar winds clear cavities around the stars and confine the ionized gas in thin shells around these cavities. The two velocity components probably originate from ionized gas at the near and the far side of these shells. The velocity distribution of both HII regions is best explained with a combination of bow shock and champagne flow models. The moving star produces a bow shock at the head of the cometary HII regions, increasing the velocity of the ionized gas compared to the systemic velocity of the neutral material. Increasing velocities of the ionized gas down both tails indicate the presence of a density gradient in the surrounding neutral material (champagne flow model). The density gradient in the southern HII region has been tentatively shown in molecular line observations of DR 21 but has to be confirmed with observations at higher spatial resolution

    Multi-epoch VLBI of a double maser super burst

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    In a rare and spectacular display, two well-known massive star forming regions, W49N and G25.65+1.05, recently underwent maser 'super burst' - their fluxes suddenly increasing above 30,000 and 18,000 Jy, respectively, reaching several orders of magnitude above their usual values. In quick-response, ToO observations with the EVN, VLBA and KaVA were obtained constituting a 4 week campaign - producing a high-cadence multi-epoch VLBI investigation of the maser emission. The combination of high-resolution, polarisation and flux monitoring during the burst provides one of the best accounts, to date, of the maser super burst phenomenon, aiding their use as astrophysical tools. These proceedings contain the preliminary results of our campaign

    CMZoom III: Spectral Line Data Release

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    We present an overview and data release of the spectral line component of the SMA Large Program, \textit{CMZoom}. \textit{CMZoom} observed 12^{12}CO(2-1), 13^{13}CO(2-1) and C18^{18}O(2-1), three transitions of H2_{2}CO, several transitions of CH3_{3}OH, two transitions of OCS and single transitions of SiO and SO, within gas above a column density of N(H2_2)1023\ge 10^{23}\,cm2^{-2} in the Central Molecular Zone (CMZ; inner few hundred pc of the Galaxy). We extract spectra from all compact 1.3\,mm \emph{CMZoom} continuum sources and fit line profiles to the spectra. We use the fit results from the H2_{2}CO 3(0,3)-2(0,2) transition to determine the source kinematic properties. We find 90\sim 90\% of the total mass of \emph{CMZoom} sources have reliable kinematics. Only four compact continuum sources are formally self-gravitating. The remainder are consistent with being in hydrostatic equilibrium assuming that they are confined by the high external pressure in the CMZ. Based on the mass and density of virially bound sources, and assuming star formation occurs within one free-fall time with a star formation efficiency of 10%75%10\% - 75\%, we place a lower limit on the future embedded star-formation rate of 0.0080.060.008 - 0.06\,M_{\odot}\,yr1^{-1}. We find only two convincing proto-stellar outflows, ruling out a previously undetected population of very massive, actively accreting YSOs with strong outflows. Finally, despite having sufficient sensitivity and resolution to detect high-velocity compact clouds (HVCCs), which have been claimed as evidence for intermediate mass black holes interacting with molecular gas clouds, we find no such objects across the large survey area.Comment: 44 pages, 41 figure

    Recent updates on the Maser Monitoring Organisation

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    The Maser Monitoring Organisation (M2O) is a research community of telescope operators, astronomy researchers and maser theoreticians pursuing a joint goal of reaching a deeper understanding of maser emission and exploring its variety of uses as tracers of astrophysical events. These proceedings detail the origin, motivations and current status of the M2O, as was introduced at the 2021 EVN symposium

    Cloud Structure of Galactic OB Cluster Forming Regions from Combining Ground and Space Based Bolometric Observations

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    We have developed an iterative procedure to systematically combine the millimeter and submillimeter images of OB cluster-forming molecular clouds, which were taken by ground based (CSO, JCMT, APEX, IRAM-30m) and space telescopes (Herschel, Planck). For the seven luminous (LL>>106^{6} LL_{\odot}) Galactic OB cluster-forming molecular clouds selected for our analyses, namely W49A, W43-Main, W43-South, W33, G10.6-0.4, G10.2-0.3, G10.3-0.1, we have performed single-component, modified black-body fits to each pixel of the combined (sub)millimeter images, and the Herschel PACS and SPIRE images at shorter wavelengths. The \sim10"" resolution dust column density and temperature maps of these sources revealed dramatically different morphologies, indicating very different modes of OB cluster-formation, or parent molecular cloud structures in different evolutionary stages. The molecular clouds W49A, W33, and G10.6-0.4 show centrally concentrated massive molecular clumps that are connected with approximately radially orientated molecular gas filaments. The W43-Main and W43-South molecular cloud complexes, which are located at the intersection of the Galactic near 3-kpc (or Scutum) arm and the Galactic bar, show a widely scattered distribution of dense molecular clumps/cores over the observed \sim10 pc spatial scale. The relatively evolved sources G10.2-0.3 and G10.3-0.1 appear to be affected by stellar feedback, and show a complicated cloud morphology embedded with abundant dense molecular clumps/cores. We find that with the high angular resolution we achieved, our visual classification of cloud morphology can be linked to the systematically derived statistical quantities (i.e., the enclosed mass profile, the column density probability distribution function, the two-point correlation function of column density, and the probability distribution function of clump/core separations)

    An Updated View of the Milky Way from Maser Astrometry

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    Astrometric observations of maser sources in the Milky Way, using the Very Long Baseline Interferometry (VLBI) technique, have been exploited to determine the spiral structure of our Galaxy. Several major spiral arms have now been pinpointed in the first and second Galactic quadrants. Fundamental Galactic parameters such as the distance to the Galactic Centre and the rotation curve and speed have been determined. In this review, we discuss the latest results from the Bar and Spiral Structure Legacy survey, the VLBI Exploration of Radio Astrometry survey and other VLBI arrays and compare them with astrometric measurements of stars from the Gaia mission. In particular, we present the peculiarities of the individual spiral arms and a thorough discussion of the methods to determine different Galactic parameters as well as the obtained values

    An intermolecular G-quadruplex as the basis for GTP recognition in the class V–GTP aptamer

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    Many naturally occurring or artificially created RNAs are capable of binding to guanine or guanine derivatives with high affinity and selectivity. They bind their ligands using very different recognition modes involving a diverse set of hydrogen bonding and stacking interactions. Apparently, the potential structural diversity for guanine, guanosine, and guanine nucleotide binding motifs is far from being fully explored. Szostak and coworkers have derived a large set of different GTP-binding aptamer families differing widely in sequence, secondary structure, and ligand specificity. The so-called class V–GTP aptamer from this set binds GTP with very high affinity and has a complex secondary structure. Here we use solution NMR spectroscopy to demonstrate that the class V aptamer binds GTP through the formation of an intermolecular two-layered G-quadruplex structure that directly incorporates the ligand and folds only upon ligand addition. Ligand binding and G-quadruplex formation depend strongly on the identity of monovalent cations present with a clear preference for potassium ions. GTP binding through direct insertion into an intermolecular G-quadruplex is a previously unobserved structural variation for ligand-binding RNA motifs and rationalizes the previously observed specificity pattern of the class V aptamer for GTP analogs