Hard X-ray and UV Observations of the 2005 January 15 Two-ribbon Flare

In this paper, we present comprehensive analysis of a two-ribbon flare observed in UV 1600{\AA} by Transition Region and Coronal Explorer and in HXRs by Reuven Ramaty High Energy Solar Spectroscopic Imager. HXR (25-100 keV) imaging observations show two kernels of size (FWHM) 15?? moving along the two UV ribbons. We find the following results. (1) UV brightening is substantially enhanced wherever and whenever the compact HXR kernel is passing, and during the HXR transit across a certain region, the UV count light curve in that region is temporally correlated with the HXR total flux light curve. After the passage of the HXR kernel, the UV light curve exhibits smooth monotonical decay. (2)We measure the apparent motion speed of the HXR sources and UV ribbon fronts, and decompose the motion into parallel and perpendicular motions with respect to the magnetic polarity inversion line (PIL). It is found that HXR kernels and UV fronts exhibit similar apparent motion patterns and speeds. The parallel motion dominates during the rise of the HXR emission, and the perpendicular motion starts and dominates at the HXR peak, the apparent motion speed being 10-40 km s-1. (3) We also find that UV emission is characterized by a rapid rise correlated with HXRs, followed by a long decay on timescales of 15-30 minutes. The above analysis provides evidence that UV brightening is primarily caused by beam heating, which also produces thick-target HXR emission. The thermal origin of UV emission cannot be excluded, but would produce weaker heating by one order of magnitude. The extended UV ribbons in this event are most likely a result of sequential reconnection along the PIL, which produces individual flux tubes (post-flare loops), subsequent non-thermal energy release and heating in these flux tubes, and then the very long cooling time of the transition region at the feet of these flux tubes.Comment: 8 figure

Disk and outflow signatures in Orion-KL: The power of high-resolution thermal infrared spectroscopy

We used the CRIRES spectrograph on the VLT to study the ro-vibrational 12CO/13CO, the Pfund beta and H2 emission between 4.59 and 4.72mu wavelengths toward the BN object, the disk candidate source n, and a proposed dust density enhancement IRC3. We detected CO absorption and emission features toward all three targets. Toward the BN object, the data partly confirm the results obtained more than 25 years ago by Scoville et al., however, we also identify several new features. While the blue-shifted absorption is likely due to outflowing gas, toward the BN object we detect CO in emission extending in diameter to ~3300AU. Although at the observational spectral resolution limit, the 13CO line width of that feature increases with energy levels, consistent with a disk origin. If one attributes the extended CO emission also to a disk origin, its extent is consistent with other massive disk candidates in the literature. For source n, we also find the blue-shifted CO absorption likely from an outflow. However, it also exhibits a narrower range of redshifted CO absorption and adjacent weak CO emission, consistent with infalling motions. We do not spatially resolve the emission for source n. For both sources we conduct a Boltzmann analysis of the 13CO absorption features and find temperatures between 100 and 160K, and H2 column densities of the order a few times 10^23cm^-2. The observational signatures from IRC3 are very different with only weak absorption against a much weaker continuum source. However, the CO emission is extended and shows wedge-like position velocity signatures consistent with jet-entrainment of molecular gas, potentially associated with the Orion-KL outflow system. We also present and discuss the Pfund beta and H2 emission in the region.Comment: 12 pages, 15 pages, accepted for A&A, you find a high-resolution copy at http://www.mpia-hd.mpg.de/homes/beuther/papers.htm

Envelope structure of deeply embedded young stellar objects in the Serpens Molecular Cloud

Aperture synthesis and single-dish (sub) millimeter molecular lines and continuum observations reveal in great detail the envelope structure of deeply embedded young stellar objects (SMM1, SMM2, SMM3, SMM4) in the densely star-forming Serpens Molecular Cloud. Resolved millimeter continuum emission constrains the density structure to a radial power law with index -2.0 +/- 0.5, and envelope masses of 8.7, 3.0, and 5.3 M_sol for SMM1, SMM3, and SMM4. The core SMM2 does not seem to have a central condensation and may not have formed a star yet. The molecular line observations can be described by the same envelope model, if an additional, small amount of warm (100 K) material is included. This probably corresponds to the inner few hundred AU of the envelope were the temperature is high. In the interferometer beam, the molecular lines reveal the inner regions of the envelopes, as well as interaction of the outflow with the surrounding envelope. Bright HCO+ and HCN emission outlines the cavities, while SiO and SO trace the direct impact of the outflow on ambient gas. Taken together, these observations provide a first comprehensive view of the physical and chemical structure of the envelopes of deeply embedded young stellar objects in a clustered environment on scales between 1000 and 10,000 AU.Comment: 46 pages, incl. 12 postscript figures, uses ApJ latex and psfig macro

Accretion and outflow structures within 1000 AU from high-mass protostars with ALMA longest baselines

Understanding the formation of massive stars is one of the unsolved problems in modern astronomy. The main difficulty is that the intense radiation from the high-luminosity stars and the thermal pressure from the resulting ionized gas (both insignificant for low-mass stars) may be able to reverse the accretion flow and prevent the star from accreting fresh material. Such feedback effects can naturally be mitigated if accretion proceeds through discs, which is the established mechanism to form sun-like stars. However, recent 3D MHD simulations have shown that accretion on 1000 au scales is through filaments rather than a large disc. This theoretical prediction has never been confirmed via observations owing to the poor linear resolution of previous studies (>1000 au). Here we present the first observational evidence that mass assembly in young high-mass stars forming in protoclusters is predominantly asymmetric and disordered. In particular, we observed the innermost regions around three deeply embedded high-mass protostars with very high spatial resolution (~100 au). We identified multiple massive (several solar masses), warm (50-150 Kelvin) filamentary streamers pointing onto the central sources, which we interpret as multi-directional accretion channels. These structures inhibit the formation of a large, steady disc. Nevertheless, the identification of fast collimated outflows in the three observed systems indicates that (non-steady) compact discs may be present (we measure upper limits on their radii of <80 for one object and <350 astronomical units for the remaining two objects). Our finding contrasts with the simplified classic paradigm of an ordered (and stable) disc/jet system and provides an experimental confirmation of a multi-directional and unsteady accretion model for massive star formation supported by recent 3D (magneto)hydrodynamic simulations.Comment: Submitted to Nature on Dec 19 2017, transferred to Nature Astronomy after review on February 8 2018, rejected after a recommendation for acceptance by one reviewer, and a more critical report by a second reviewer. To be submitted to ApJ. Comments from colleagues (even critical ones) are welcom

Carbon in different phases ([CII], [CI], and CO) in infrared dark clouds: Cloud formation signatures and carbon gas fractions

Context: How do molecular clouds form out of the atomic phase? And what are the relative fractions of carbon in the ionized, atomic and molecular phase? These are questions at the heart of cloud and star formation. Methods: Using multiple observatories from Herschel and SOFIA to APEX and the IRAM 30m telescope, we mapped the ionized, atomic and molecular carbon ([CII]@1900GHz, [CI]@492GHz and C18O(2-1)@220GHz) at high spatial resolution (12"-25") in four young massive infrared dark clouds (IRDCs). Results: The three carbon phases were successfully mapped in all four regions, only in one source the [CII] line remained a non-detection. Both the molecular and atomic phases trace the dense structures well, with [CI] also tracing material at lower column densities. [CII] exhibits diverse morphologies in our sample, from compact to diffuse structures probing the cloud environment. In at least two out of the four regions, we find kinematic signatures strongly indicating that the dense gas filaments have formed out of a dynamically active and turbulent atomic/molecular cloud, potentially from converging gas flows. The atomic-to-molecular carbon gas mass ratios are low between 7% and 12% with the lowest values found toward the most quiescent region. In the three regions where [CII] is detected, its mass is always higher by a factor of a few than that of the atomic carbon. The ionized carbon emission depends as well on the radiation field, however, we also find strong [CII] emission in a region without significant external sources, indicating that other processes, e.g., energetic gas flows can contribute to the [CII] excitation as well.Comment: 15 pages, 18 figures, accepted by Astronomy & Astrophysics, a higher resolution version can be found at http://www.mpia.de/homes/beuther/papers.htm

The warm and dense Galaxy - tracing the formation of dense cloud structures out to the Galactic Center

The past two decades have seen extensive surveys of the far-infrared to submillimeter continuum emission in the plane of our Galaxy. We line out prospects for the coming decade for corresponding molecular and atomic line surveys which are needed to fully understand the formation of the dense structures that give birth to clusters and stars out of the diffuse interstellar medium. We propose to work towards Galaxy wide surveys in mid-J CO lines to trace shocks from colliding clouds, Galaxy-wide surveys for atomic Carbon lines in order to get a detailed understanding of the relation of atomic and molecular gas in clouds, and to perform extensive surveys of the structure of the dense parts of molecular clouds to understand the importance of filaments/fibers over the full range of Galactic environments and to study how dense cloud cores are formed from the filaments. This work will require a large (50m) Single Dish submillimeter telescope equipped with massively multipixel spectrometer arrays, such as envisaged by the AtLAST project.Comment: Science white paper submitted to the Astro2020 Decadal Surve

The Ultraviolet Attenuation Law in Backlit Spiral Galaxies

(Abridged) The effective extinction law (attenuation behavior) in galaxies in the emitted ultraviolet is well known only for actively star-forming objects and combines effects of the grain properties, fine structure in the dust distribution, and relative distributions of stars and dust. We use GALEX, XMM Optical Monitor, and HST data to explore the UV attenuation in the outer parts of spiral disks which are backlit by other UV-bright galaxies, starting with candidates provided by Galaxy Zoo participants. Our analysis incorporates galaxy symmetry, using non-overlapping regions of each galaxy to derive error estimates on the attenuation measurements. The entire sample has an attenuation law close to the Calzetti et al. (1994) form; the UV slope for the overall sample is substantially shallower than found by Wild et al. (2011), a reasonable match to the more distant galaxies in our sample but not to the weighted combination including NGC 2207. The nearby, bright spiral NGC 2207 alone gives accuracy almost equal to the rest of our sample, and its outer arms have a very low level of foreground starlight. This "grey" law can be produced from the distribution of dust alone, without a necessary contribution from differential escape of stars from dense clouds. The extrapolation needed to compare attenution between backlit galaxies at moderate redshifts, and local systems from SDSS data, is mild enough to allow use of galaxy overlaps to trace the cosmic history of dust. For NGC 2207, the covering factor of clouds with small optical attenuation becomes a dominant factor farther into the ultraviolet, which opens the possibility that widespread diffuse dust dominates over dust in star-forming regions deep into the ultraviolet. Comparison with published radiative-transfer models indicates that the role of dust clumping dominates over differences in grain populations, at this spatial resolution.Comment: In press, Astronomical Journa