6,497 research outputs found

    Unveiling Sources of Heating in the Vicinity of the Orion BN/KL Hot Core as Traced by Highly Excited Inversion Transitions of Ammonia

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    Using the Expanded Very Large Array, we have mapped the vicinity of the Orion BN/KL Hot Core with sub-arcsecond angular resolution in seven metastable inversion transitions of ammonia: (J,K)=(6,6) to (12,12). This emission comes from levels up to 1500 K above the ground state, enabling identification of source(s) responsible for heating the region. We used this multi-transition dataset to produce images of the rotational/kinetic temperature and the column density of ammonia for ortho and para species separately and on a position-by-position basis. We find rotational temperature and column density in the range 160-490 K and (1-4)x10^17 cm^-2, respectively. Our spatially-resolved images show that the highest (column) density and hottest gas is found in a northeast-southwest elongated ridge to the southeast of Source I. We have also measured the ortho-para ratio of ammonia, estimated to vary in the range 0.9-1.6. Enhancement of ortho with respect to para and the offset of hot ammonia emission peaks from known (proto)stellar sources provide evidence that the ammonia molecules have been released from dust grains into the gas-phase through the passage of shocks and not by stellar radiation. We propose that the combined effect of Source I's proper motion and its low-velocity outflow impinging on a pre-existing dense medium is responsible for the excitation of ammonia and the Orion Hot Core. Finally, we found for the first time evidence of a slow (5 km/s) and compact (1000 AU) outflow towards IRc7.Comment: To appear in Astrophysical Journal Letters Special Issue on the EVLA. 8 pages, 4 figure

    A Documentary of High-Mass Star Formation: Probing the Dynamical Evolution of Orion Source I on 10-100 AU Scales using SiO Masers

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    A comprehensive picture of high-mass star formation has remained elusive, in part because examples of high-mass YSOs tend to be relatively distant, deeply embedded, and confused with other emission sources. These factors have impeded dynamical investigations within tens of AU of high-mass YSOs--scales that are critical for probing the interfaces where outflows from accretion disks are launched and collimated. Using observations of SiO masers obtained with the VLA and the VLBA, the KaLYPSO project is overcoming these limitations by mapping the structure and dynamical/temporal evolution of the material 10-1000 AU from the nearest high-mass YSO: Radio Source I in the Orion BN/KL region. Our data include ~40 epochs of VLBA observations over a several-year period, allowing us to track the proper motions of individual SiO maser spots and to monitor changes in the physical conditions of the emitting material with time. Ultimately these data will provide 3-D maps of the outflow structure over approximately 30% of the outflow crossing time. Here we summarize recent results from the KaLYPSO project, including evidence that high-mass star formation is occurring via disk-mediated accretion.Comment: 5 pages; to appear in the proceedings of IAU Symposium 242, Astrophysical Masers and their Environments, ed. J. Chapman & W. Baa

    Toward a New Distance to the Active Galaxy NGC 4258: II. Centripetal Accelerations and Investigation of Spiral Structure

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    We report measurements of centripetal accelerations of maser spectral components of NGC 4258 for 51 epochs spanning 1994 to 2004. This is the second paper of a series, in which the goal is determination of a new geometric maser distance to NGC 4258 accurate to possibly ~3%. We measure accelerations using a formal analysis method that involves simultaneous decomposition of maser spectra for all epochs into multiple, Gaussian components. Components are coupled between epochs by linear drifts (accelerations) from their centroid velocities at a reference epoch. For high-velocity emission, accelerations lie in the range -0.7 to +0.7 km/s/yr indicating an origin within 13 degrees of the disk midline (the perpendicular to the line-of-sight to the black hole). Comparison of high-velocity emission projected positions in VLBI images, with those derived from acceleration data, provides evidence that masers trace real gas dynamics. High-velocity emission accelerations do not support a model of trailing shocks associated with spiral arms in the disk. However, we find strengthened evidence for spatial periodicity in high-velocity emission, of wavelength 0.75 mas. This supports suggestions of spiral structure due to density waves in the nuclear accretion disk of an active galaxy. Accelerations of low-velocity (systemic) emission lie in the range 7.7 to 8.9 km/s/yr, consistent with emission originating from a concavity where the thin, warped disk is tangent to the line-of-sight. A trend in accelerations of low-velocity emission as a function of Doppler velocity may be associated with disk geometry and orientation, or with the presence of spiral structure.Comment: Accepted to ApJ, 48 pages and 20 figure

    The Truth in Compatibilism and the truth of Libertarianism

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    The paper offers the outlines of a response to the often-made suggestion is that it is impossible to see how indeterminism could possibly provide us with anything that we might want in the way of freedom, anything that could really amount to control, as opposed merely to an openness in the flow of reality that would constitute merely the injection of chance, or randomness, into the unfolding of the processes which underlie our activity. It is suggested that the best first move for the libertarian is to make a number of important concessions to the compatibilist. It should be conceded, in particular, that certain sorts of alternative possibilities are neither truly available to real, worldly agents, nor required in order that those agents should act freely; and it should be admitted also that it is the compatibilist who tends to give the most plausible sorts of analyses of many of the ‘can’ and ‘could have’ statements which seem to need to be assertible of those agents we regard as free. But these concessions do not bring compatibilism itself in their wake. The most promising version of libertarianism, it is argued, should be based on the idea that agency itself (and not merely some special instances of it which we might designate with the honorific appellation ‘free’) is inconsistent with determinism. This version of libertarianism, it is claimed, can avoid the objection that indeterminism is as difficult to square with true agential control as determinism can sometimes seem to be

    VLBA imaging of the 3mm SiO maser emission in the disk-wind from the massive protostellar system Orion Source I

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    We present the first images of the 28SiO v=1, J=2-1 maser emission around the closest known massive young stellar object Orion Source I observed at 86 GHz (3mm) with the VLBA. These images have high spatial (~0.3 mas) and spectral (~0.054 km/s) resolutions. We find that the 3mm masers lie in an X-shaped locus consisting of four arms, with blue-shifted emission in the south and east arms and red-shifted emission in the north and west arms. Comparisons with previous images of the 28SiO v=1,2, J=1-0 transitions at 7mm (observed in 2001-2002) show that the bulk of the J=2-1 transition emission follows the streamlines of the J=1-0 emission and exhibits an overall velocity gradient consistent with the gradient at 7mm. While there is spatial overlap between the 3mm and 7mm transitions, the 3mm emission, on average, lies at larger projected distances from Source I (~44 AU compared with ~35 AU for 7mm). The spatial overlap between the v=1, J=1-0 and J=2-1 transitions is suggestive of a range of temperatures and densities where physical conditions are favorable for both transitions of a same vibrational state. However, the observed spatial offset between the bulk of emission at 3mm and 7mm possibly indicates different ranges of temperatures and densities for optimal excitation of the masers. We discuss different maser pumping models that may explain the observed offset. We interpret the 3mm and 7mm masers as being part of a single wide-angle outflow arising from the surface of an edge-on disk rotating about a northeast-southwest axis, with a continuous velocity gradient indicative of differential rotation consistent with a Keplerian profile in a high-mass proto-binary.Comment: 11 pages, 12 figures; accepted for publication in A&
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