110 research outputs found
Discovery of superthermal hydroxyl (OH) in the HH211 outflow
We present a 5-37 micron infrared spectrum obtained with the Spitzer Space
Telescope toward the southeastern lobe of the young protostellar outflow HH211.
The spectrum shows an extraordinary sequence of OH emission lines arising in
highly excited rotational levels up to an energy E/k~28200K above the ground
level. This is, to our knowledge, by far the highest rotational excitation of
OH observed outside Earth. The spectrum also contains several pure rotational
transitions of H2O (v=0), H2 (v=0) S(0) to S(7), HD (v=0) R(3) to R(6), and
atomic fine-structure lines of [Fe II], [Si II], [Ne II], [S I], and [Cl I].
The origin of the highly excited OH emission is most likely the
photodissociation of H2O by the UV radiation generated in the terminal outflow
shock of HH211.Comment: ApJ Letters, in pres
Land Surface Temperature Patterns in the Urban Agglomeration of Krakow (Poland) Derived from Landsat-7/ETM+ Data
The aim of this study was to identify typical and specific features of land surface temperature (LST) distribution in the city of Krakow and its surroundings with the use of Landsat/ETM+ data. The paper contains a detailed description of the study area and technical properties of the Landsat program and data, as well as a complete methodology of LST retrieval. Retrieved LST records have been standardized in order to ensure comparability between satellite images acquired during different seasons. The method also enables identification of characteristic thermal regions, i.e. areas always colder and always warmer than a zonal mean LST value for Krakow. The research includes spatial analysis of the standardized LST with regard to different land cover types. Basic zonal statistics such as mean standardized LST and percentage share of hot and cold regions within 10 land cover types were calculated. GIS was used for automated data processing and mapping. The results confirmed the most obvious dependence of the LST on different land cover types. Some more factors influencing the LST were recognized on the basis of detailed investigation of the LST pattern in the urban agglomeration of Krakow. The factors are: emission of anthropogenic heat, insolation of the surfaces depending first of all on land relief and shape of buildings, seasonal changes of vegetation and weather conditions at the time of satellite image acquisition
The Brightening of Re50N: Accretion Event or Dust Clearing?
The luminous Class I protostar HBC 494, embedded in the Orion A cloud, is
associated with a pair of reflection nebulae, Re50 and Re50N, which appeared
sometime between 1955 and 1979. We have found that a dramatic brightening of
Re50N has taken place sometime between 2006 and 2014. This could result if the
embedded source is undergoing a FUor eruption. However, the near-infrared
spectrum shows a featureless very red continuum, in contrast to the strong CO
bandhead absorption displayed by FUors. Such heavy veiling, and the high
luminosity of the protostar, is indicative of strong accretion but seemingly
not in the manner of typical FUors. We favor the alternative explanation that
the major brightening of Re50N and the simultaneous fading of Re50 is caused by
curtains of obscuring material that cast patterns of illumination and shadows
across the surface of the molecular cloud. This is likely occurring as an
outflow cavity surrounding the embedded protostar breaks through to the surface
of the molecular cloud. Several Herbig-Haro objects are found in the region.Comment: 8 pages, accepted by Ap
Land Surface Temperature Patterns in the Urban Agglomeration of Krakow (Poland) Derived from Landsat-7/ETM+ Data
A shallow though extensive H2 2.12 micron imaging survey of Taurus-Auriga-Perseus: I. NGC1333, L1455, L1448 and B1
We discuss wide-field near-IR imaging of the NGC1333, L1448, L1455 and B1
star forming regions in Perseus. The observations have been extracted from a
much larger narrow-band imaging survey of the Taurus-Auriga-Perseus complex.
These H2 2.12 micron observations are complemented by broad-band K imaging,
mid-IR imaging and photometry from the Spitzer Space Telescope, and published
submillimetre CO J=3-2 maps of high-velocity molecular outflows. We detect and
label 85 H2 features and associate these with 26 molecular outflows. Three are
parsec-scale flows, with a mean flow lobe length exceeding 11.5 arcmin. 37
(44%) of the detected H2 features are associated with a known Herbig-Haro
object, while 72 (46%) of catalogued HH objects are detected in H2 emission.
Embedded Spitzer sources are identified for all but two of the 26 molecular
outflows. These candidate outflow sources all have high near-to-mid-IR spectral
indices (mean value of alpha ~ 1.4) as well as red IRAC 3.6-4.5 micron and
IRAC/MIPS 4.5-24.0 micron colours: 80% have [3.6]-[4.5] > 1.0 and [4.5]-[24] >
1.5. These criteria - high alpha and red [4.5]-[24] and [3.6]-[4.5] colours -
are powerful discriminants when searching for molecular outflow sources.
However, we find no correlation between alpha and flow length or opening angle,
and the outflows appear randomly orientated in each region. The more massive
clouds are associated with a greater number of outflows, which suggests that
the star formation efficiency is roughly the same in each region.Comment: 23 pages (including Appoendix); 11 main text figures, 5 colour
appendix figs uploaded as gifs; accepted by MNRAS; for higher-resolution
figures please visit http://www.jach.hawaii.edu/~cdavis
Comparison of Magnetic Field Structures on Different Scales in and around the Filamentary Dark Cloud GF 9
New visible polarization data combined with existing IR and FIR polarization
data are used to study how the magnetic field threading the filamentary
molecular cloud GF 9 connects to larger structures in its general environment.
We find that when both visible and NIR polarization data are plotted as a
function of extinction, there is no evidence for a plateau or a saturation
effect in the polarization at Av ~ 1.3 as seen in dark clouds in Taurus. This
lack of saturation effect suggests that even in the denser parts of GF 9 we are
still probing the magnetic field. The visible polarization is smooth and has a
well-defined orientation. The IR data are also well defined but with a
different direction, and the FIR data in the core region are well defined and
with yet another direction, but are randomly distributed in the filament
region. On the scale of a few times the mean radial dimension of the molecular
cloud, it is as if the magnetic field were `blind' to the spatial distribution
of the filaments while on smaller scales within the cloud, in the core region
near the IRAS point source PSC 20503+6006, polarimetry shows a rotation of the
magnetic field lines in these denser phases. Hence, in spite of the fact that
the spatial resolution is not the same in the visible/NIR and in the FIR data,
all the data put together indicate that the field direction changes with the
spatial scale. Finally, the Chandrasekhar and Fermi method is used to evaluate
the magnetic field strength, indicating that the core region is approximately
magnetically critical. A global interpretation of the results is that in the
core region an original poloidal field could have been twisted by a rotating
elongated (core+envelope) structure. There is no evidence for turbulence and
ambipolar diffusion does not seem to be effective at the present time.Comment: 33 pages, 6 tables, 8 figures, Accepted by Ap
Satellite-based trends of solar radiation and cloud parameters in Europe
Solar radiation is the main driver of the Earth\u2019s climate. Measuring solar radiation and analysing its
interaction with clouds are essential for the understanding of the climate system. The EUMETSAT Satellite Application
Facility on Climate Monitoring (CM SAF) generates satellite-based, high-quality climate data records,
with a focus on the energy balance and water cycle. Here, multiple of these data records are analyzed in a common
framework to assess the consistency in trends and spatio-temporal variability of surface solar radiation,
top-of-atmosphere reflected solar radiation and cloud fraction. This multi-parameter analysis focuses on Europe
and covers the time period from 1992 to 2015. A high correlation between these three variables has been found
over Europe. An overall consistency of the climate data records reveals an increase of surface solar radiation and
a decrease in top-of-atmosphere reflected radiation. In addition, those trends are confirmed by negative trends
in cloud cover. This consistency documents the high quality and stability of the CM SAF climate data records,
which are mostly derived independently from each other. The results of this study indicate that one of the main
reasons for the positive trend in surface solar radiation since the 1990\u2019s is a decrease in cloud coverage even if
an aerosol contribution cannot be completely ruled out
And in the Darkness Bind Them: Equatorial Rings, B[e] Supergiants, and the Waists of Bipolar Nebulae
We report the discovery of two new circumstellar ring nebulae in the western
Carina Nebula. The brighter object, SBW1, resembles a lidless staring eye and
encircles a B1.5 Iab supergiant. Its size is identical to the inner ring around
SN1987A, but SBW1's low N abundance indicates that the star didn't pass through
a RSG phase. The fainter object, SBW2, is a more distorted ring, is N-rich, and
has a central star that seems to be invisible. We discuss these two new nebulae
in context with rings around SN1987A, Sher25, HD168625, RY Scuti, WeBo1, SuWt2,
and others. The ring bearers fall into two groups: Five rings surround hot
supergiants, and all except for the one known binary are carbon copies of the
ring around SN1987A. We propose a link between these rings and B[e]
supergiants, where the rings derive from the same material in an earlier B[e]
phase. The remaining four rings surround evolved intermediate-mass stars; all
members of this ring fellowship are close binaries, hinting that binary
interactions govern the forging of such rings. We estimate that there may be
several thousand more dark rings in the Galaxy, but we are scarcely aware of
their existence due to selection effects. The lower-mass objects might be the
equatorial density enhancements often invoked to bind the waists of bipolar
PNe.Comment: AJ accepted, 27 page
Star formation in Perseus - V. Outflows detected by HARP
Molecular outflows provide an alternative method of identifying protostellar
cores, complementary to recent mid-infrared studies. Continuing our studies of
Perseus, we investigate whether all Spitzer-identified protostars, and
particularly those with low luminosities, drive outflows, and if any new
protostellar cores (perhaps harbouring low-mass sources) can be identified via
their outflows alone. We have used the heterodyne array receiver HARP on JCMT
to make deep 12CO 3-2 maps of submm cores in Perseus, extending and deepening
our earlier study with RxB and bringing the total number of SCUBA cores studied
up to 83. Our survey includes 23/25 of the Dunham et al. (2008) Spitzer
low-luminosity objects believed to be embedded protostars, including three
VeLLOs. All but one of the cores identified as harbouring embedded YSOs have
outflows, confirming outflow detections as a good method for identifying
protostars. We detect outflows from 20 Spitzer low-luminosity objects. We do
not conclusively detect any outflows from IR-quiet cores, though confusion in
clustered regions such as NGC1333 makes it impossible to identify all the
individual driving sources. This similarity in detection rates despite the
difference in search methods and detection limits suggests either that the
sample of protostars in Perseus is now complete, or that the existence of an
outflow contributes to the Spitzer detectability, perhaps through the
contribution of shocked H2 emission in the IRAC bands. For five of the
low-luminosity sources, there is no protostellar envelope detected at 350
microns and the Spitzer emission is entirely due to shocks. Additionally, we
detect the outflow from IRAS 03282+3035 at 850 microns with SCUBA due to CO
line contamination in the continuum passband.Comment: 25 pages, 12 figures, to be published in A&
Simultaneous X-ray and radio observations of Young Stellar Objects in NGC 1333 and IC 348
Young Stellar Objects (YSOs) and in particular protostars are known to show a
variety of high-energy processes. Observations in the X-ray and centimetric
radio wavelength ranges are thought to constrain some of these processes, e.g.,
coronal-type magnetic activity. There is a well-known empirical correlation of
radio and X-ray luminosities in active stars, the so-called Guedel-Benz
relation. Previous evidence whether YSOs are compatible with this relation
remains inconclusive for the earliest evolutionary stages. The main difficulty
is that due to the extreme variability of these sources, simultaneous
observations are essential. Until now, only few YSOs and only a handful of
protostars have been observed simultaneously in the X-ray and radio range. To
expand the sample, we have obtained such observations of two young clusters
rich in protostars, NGC 1333 and IC 348. While the absolute sensitivity is
lower for these regions than for more nearby clusters like CrA, we find that
even in deep continuum observations carried out with the NRAO Very Large Array,
the radio detection fraction for protostars in these clusters is much lower
than the X-ray detection fraction. Very few YSOs are detected in both bands,
and we find the radio and X-ray populations among YSOs to be largely distinct.
We combine these new results with previous simultaneous Chandra and VLA
observations of star-forming regions and find that YSOs with detections in both
bands appear to be offset toward higher radio luminosities for given X-ray
luminosities when compared to the Guedel-Benz relation, although even in this
sensitive dataset most sources are too weak for the radio detections to provide
information on the emission processes. The considerably improved sensitivity of
the Expanded Very Large Array will provide a better census of the YSO radio
population as well as better constraints on the emission mechanisms.Comment: Accepted for publication in Ap
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