44 research outputs found
Compactification of a Drinfeld Period Domain over a Finite Field
We study a certain compactification of the Drinfeld period domain over a
finite field which arises naturally in the context of Drinfeld moduli spaces.
Its boundary is a disjoint union of period domains of smaller rank, but these
are glued together in a way that is dual to how they are glued in the
compactification by projective space. This compactification is normal and
singular along all boundary strata of codimension . We study its geometry
from various angles including the projective coordinate ring with its Hilbert
function, the cohomology of twisting sheaves, the dualizing sheaf, and give a
modular interpretation for it. We construct a natural desingularization which
is smooth projective and whose boundary is a divisor with normal crossings. We
also study its quotients by certain finite groups
Atomic Carbon in M82: Physical conditions derived from simultaneous observations of the [CI] fine structure submillimeter wave transitions
We report the first extragalactic detection of the neutral carbon [CI]
3P2-3P1 fine structure line at 809 GHz. The line was observed towards M82
simultaneously with the 3P1-3P0 line at 492 GHz, providing a precise
measurement of the J=2-1/J=1-0 integrated line ratio of 0.96 (on a [K km s^-1]
-scale). This ratio constrains the [CI] emitting gas to have a temperature of
at least 50 K and a density of at least 10^4 cm^-3. Already at this minimum
temperature and density, the beam averaged CI-column density is large, 2.1
10^18 cm^-2, confirming the high CI/CO abundance ratio of approximately 0.5
estimated earlier from the 492 GHz line alone. We argue that the [CI] emission
from M82 most likely arises in clouds of linear size around a few pc with a
density of about 10^4 cm^-3 or slightly higher and temperatures of 50 K up to
about 100 K.Comment: 4 pages, 2 figures, ApJL in press, postscript also available at
ftp://apollo.ph1.uni-koeln.de/pub/stutzki/m82_pap.ps.gz
e-mail-contact:[email protected]
The Antarctic Submillimeter Telescope and Remote Observatory (AST/RO)
AST/RO, a 1.7 m diameter telescope for astronomy and aeronomy studies at
wavelengths between 200 and 2000 microns, was installed at the South Pole
during the 1994-1995 Austral summer. The telescope operates continuously
through the Austral winter, and is being used primarily for spectroscopic
studies of neutral atomic carbon and carbon monoxide in the interstellar medium
of the Milky Way and the Magellanic Clouds. The South Pole environment is
unique among observatory sites for unusually low wind speeds, low absolute
humidity, and the consistent clarity of the submillimeter sky. Four heterodyne
receivers, an array receiver, three acousto-optical spectrometers, and an array
spectrometer are installed. A Fabry-Perot spectrometer using a bolometric array
and a Terahertz receiver are in development. Telescope pointing, focus, and
calibration methods as well as the unique working environment and logistical
requirements of the South Pole are described.Comment: 57 pages, 15 figures. Submitted to PAS
Photon Dominated Regions in NGC 3603
Aims: We aim at deriving the excitation conditions of the interstellar gas as
well as the local FUV intensities in the molecular cloud surrounding NGC 3603
to get a coherent picture of how the gas is energized by the central stars.
Methods: The NANTEN2-4m submillimeter antenna is used to map the [CI] 1-0, 2-1
and CO 4-3, 7-6 lines in a 2' x 2' region around the young OB cluster NGC 3603
YC. These data are combined with C18O 2-1 data, HIRES-processed IRAS 60 and 100
micron maps of the FIR continuum, and Spitzer/IRAC maps. Results: The NANTEN2
observations show the presence of two molecular clumps located south-east and
south-west of the cluster and confirm the overall structure already found by
previous CS and C18O observations. We find a slight position offset of the peak
intensity of CO and [CI], and the atomic carbon appears to be further extended
compared to the molecular material. We used the HIRES far-infrared dust data to
derive a map of the FUV field heating the dust. We constrain the FUV field to
values of \chi = 3 - 6 \times 10^3 in units of the Draine field across the
clouds. Approximately 0.2 to 0.3 % of the total FUV energy is re-emitted in the
[CII] 158 {\mu}m cooling line observed by ISO. Applying LTE and escape
probability calculations, we derive temperatures (TMM1 = 43 K, TMM2 = 47 K),
column densities (N(MM1) = 0.9 \times 10^22 cm^-2, N(MM2) = 2.5 \times 10^22
cm^-2) and densities (n(MM1) = 3 \times 10^3 cm^-3, n(MM2) = 10^3 -10^4 cm^-3)
for the two observed molecular clumps MM1 and MM2. Conclusions: The cluster is
strongly interacting with the ambient molecular cloud, governing its structure
and physical conditions. A stability analysis shows the existence of
gravitationally collapsing gas clumps which should lead to star formation.
Embedded IR sources have already been observed in the outskirts of the
molecular cloud and seem to support our conclusions.Comment: 13 pages, 10 figures, accepted for publication by A&
The origin of the [C II] emission in the S140 PDRs - new insights from HIFI
Using Herschel's HIFI instrument we have observed [C II] along a cut through
S140 and high-J transitions of CO and HCO+ at two positions on the cut,
corresponding to the externally irradiated ionization front and the embedded
massive star forming core IRS1. The HIFI data were combined with available
ground-based observations and modeled using the KOSMA-tau model for photon
dominated regions. Here we derive the physical conditions in S140 and in
particular the origin of [C II] emission around IRS1. We identify three
distinct regions of [C II] emission from the cut, one close to the embedded
source IRS1, one associated with the ionization front and one further into the
cloud. The line emission can be understood in terms of a clumpy model of
photon-dominated regions. At the position of IRS1, we identify at least two
distinct components contributing to the [C II] emission, one of them a small,
hot component, which can possibly be identified with the irradiated outflow
walls. This is consistent with the fact that the [C II] peak at IRS1 coincides
with shocked H2 emission at the edges of the outflow cavity. We note that
previously available observations of IRS1 can be well reproduced by a
single-component KOSMA-tau model. Thus it is HIFI's unprecedented spatial and
spectral resolution, as well as its sensitivity which has allowed us to uncover
an additional hot gas component in the S140 region.Comment: accepted for publication in Astronomy and Astrophysics (HIFI special
issue
Clumpy photon-dominated regions in Carina. I. [CI] and mid-J CO lines in two 4'x4' fields
The Carina region is an excellent astrophysical laboratory for studying the
feedback mechanisms of newly born, very massive stars within their natal giant
molecular clouds (GMCs) at only 2.35 kpc distance. We use a clumpy PDR model to
analyse the observed intensities of atomic carbon and CO and to derive the
excitation conditions of the gas. The NANTEN2-4m submillimeter telescope was
used to map the [CI] 3P1-3P0, 3P2-3P1 and CO 4-3, 7-6 lines in two 4'x4'
regions of Carina where molecular material interfaces with radiation from the
massive star clusters. One region is the northern molecular cloud near the
compact OB cluster Tr14, and the second region is in the molecular cloud south
of etaCar and Tr16. These data were combined with 13CO SEST spectra, HIRES/IRAS
60um and 100um maps of the FIR continuum, and maps of 8um IRAC/Spitzer and MSX
emission. We used the HIRES far-infrared dust data to create a map of the FUV
field heating the gas. The northern region shows an FUV field of a few 1000 in
Draine units while the field of the southern region is about a factor 10
weaker. We constructed models consisting of an ensemble of small spherically
symmetric PDR clumps within the 38" beam (0.43pc), which follow canonical
power-law mass and mass-size distributions. We find that an average local clump
density of 2x10**5 cm-3 is needed to reproduce the observed line emission at
two selected interface positions. Stationary, clumpy PDR models reproduce the
observed cooling lines of atomic carbon and CO at two positions in the Carina
Nebula.Comment: accepted by A&
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Picard-Lefschetz Oscillators for the Drinfeld-Lafforgue-Vinberg Compactification
We study the singularities of the Drinfeld-Lafforgue-Vinberg compactification of the moduli stack of G-bundles on a smooth projective curve for a reductive group G. The study of these compactifications was initiated by V. Drinfeld (for G=GL_2) and continued by L. Lafforgue (for G=GL_n) in their work on the Langlands correspondence for function fields; unlike the work of Drinfeld and Lafforgue, however, we focus on questions about the singularities of these compactifications which arise naturally in the geometric Langlands program. A definition of the compactification for a general reductive group G is also due to Drinfeld (unpublished) and relies on the Vinberg semigroup of G; this case will be dealt with in the forthcoming work [Sch]. In the present work we focus on the case G=SL_2. In this case the compactification can alternatively be viewed as a canonical one-parameter degeneration of the moduli space of SL_2-bundles. We study the singularities of this one-parameter degeneration via the weight-monodromy theory of the associated nearby cycles construction: We give an explicit description of the nearby cycles sheaf together with its monodromy action in terms of certain novel perverse sheaves which we call "Picard-Lefschetz oscillators", and then use this description to determine the intersection cohomology sheaf and other invariants of the singularities. Our proofs rely on the construction of certain local models for the one-parameter degeneration which themselves form one-parameter families of spaces which are factorizable in the sense of Beilinson and Drinfeld. We also include a first application on the level of functions.Mathematic