74 research outputs found
Measuring the Slope of the Dust Extinction Law and the Power Spectrum of Dust Clouds Using Differentially-Reddened Globular Clusters
We present 3 methods for measuring the Galactic dust extinction law slope R_V
and a method for quantifying fine structure in dust clouds in the direction of
differentially-reddened globular clusters. We apply these methods to the low-
latitude globular cluster NGC4833 which displays variable extinction/reddening
about a mean ~ 1. A set of Monte Carlo simulations is used to
characterize the efficacy of the methods. The essence of the first 2 methods is
to determine, for an assumed value of R_V, the relative visual extinction
delta(A_V) of each cluster horizontal branch (HB) star with respect to an
empirical HB locus; the locus is derived from the CMD of a subset of stars near
the cluster center in which differential reddening is small. A star-by-star
comparison of delta(A_V) from the (B-V,V) CMD with that from the (V-I,V) CMD is
used to find the optimal R_V. In the third method, R_V is determined by
minimizing the scatter in the HB in the (B-V, V) CMD after correcting the
photometry for extinction and reddening using the Schlegel et al. (1998) dust
maps. The weighted average of the results from 3 methods gives R_V = 3.0 +/-
0.4 for the dust towards NGC4833. The dust fine structure is quantified via the
difference, Delta(A_V)_ij = [delta(A_V)]_i - [delta(A_V)]_j, between pairs of
cluster HB stars (i,j) as a function of their angular separation r_ij. The
variance of Delta(A_V)_ij is found to have a power- law dependence on angular
scale: var(r) \propto r^(+0.9 +/- 0.1). This translates into an angular power
spectrum P(kappa) \propto kappa^(-1.9 +/- 0.1) for r ~ 1' - 5', where kappa =
1/r. The dust angular power spectrum on small scales (from optical data)
matches smoothly onto the larger-scale power spectrum derived from Schlegel et
al.'s far-infrared map of the dust thermal emission.Comment: 36 pages, 15 figures, 1 table. Accepted for publication in the
Astronomical Journal (July 2004
A search for interstellar bubbles surrounding massive stars in Perseus OB1
We have examined the interstellar medium in the vicinity of massive stars belonging to the Per OB1 association based on neutral hydrogen 21 cm observations obtained with the 100 m radio telescope at Effelsberg (HPBW = 8′.4) and complementary data from the Leiden-Dwingeloo H I Survey (HPBW = 36′). The higher angular resolution H I observations allowed us to discover probable wind-blown bubbles related to four massive stars in the association, namely, HD 14442 [O5n(f)p], HD 14947 [O5If+], HD 13022 [O9.5II-III((n))], and HD 13338 [O9.5V], while the detection of a wind-blown bubble associated with HD 16691 [O5If+] is less conclusive. A clear H I shell coincident in position with two B1III stars (HD 15233 and Hilt 311) was also detected. Some of these features also have infrared and/or molecular counterparts. The energetics of the structures related to each massive star is analyzed. The new H I interstellar bubbles appear to be similar to the ones found surrounding Wolf-Rayet stars and other Of stars. The large-scale maps obtained using the lower angular resolution H I data show that most of the early-type stars belonging to Per OB1 are placed in a region of low H I emission. The association could have blown a H I shell of about 350 Ă— 550 pc in size. This large H I shell has an infrared counterpart.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto Argentino de RadioastronomĂ
The Composition of the Interstellar Medium towards the Lockman Hole. HI, UV and X-ray observations
The Lockman Hole is well known as the region with the lowest neutral atomic
hydrogen colum density on the entire sky. We present an analysis of the soft
X-ray background radiation towards the Lockman Hole using ROSAT all-sky survey
data. This data is correlated with the Leiden/Dwingeloo survey (Galactic HI
21cm-line emission) in order to model the soft X-ray background by using
radiative transfer calculations for four ROSAT energy bands simultaneously. It
turns out, that an important gas fraction, ranging between 20-50%,of the X-ray
absorbing material is not entirely traced by the HI but is in the form of
ionized hydrogen. Far-ultraviolet absorption line measurements by FUSE are
consistent with this finding and support an ionized hydrogen component towards
the Lockman Hole.Comment: 11 pages, 11 figures, Accepted for Publication in Astronomy &
Astrophysics, For full resolution images, see
http://www.astro.uni-bonn.de/~mkappes/pub/ms3506.pd
A FUSE survey of high-latitude Galactic molecular hydrogen
Measurements of molecular hydrogen (H_2) column densities are presented for
the first six rotational levels (J=0 to 5) for 73 extragalactic targets
observed with FUSE. All of these have a final signal-to-noise ratio larger than
\snlimit, and are located at galactic latitude |b|>20 deg. The individual
observations were calibrated with the FUSE calibration pipeline CalFUSE version
2.1 or higher, and then carefully aligned in velocity. The final velocity
shifts for all the FUSE segments are listed. H_2 column densities or limits are
determined for the 6 lowest rotational (J) levels for each HI component in the
line of sight, using a curve-of-growth approach at low column densities ~16.5),
and Voigt-profile fitting at higher column densities. Detections include 73
measurements of low-velocity H_2 in the Galactic Disk and lower Halo. Eight
sightlines yield non-detections for Galactic H_2. The measured column densities
range from log N(H_2)=14 to log N(H_2)=20. Strong correlations are found
between log N(H_2) and T_01, the excitation temperature of the H_2, as well as
between log N(H_2) and the level population ratios (log (N(J')/N(J))). The
average fraction of nuclei in molecular hydrogen (f(H_2)) in each sightline is
calculated; however, because there are many HI clouds in each sightline, the
physics of the transition from HI to H_2 can not be studied. Detections also
include H2 in 16 intermediate-velocity clouds in the Galactic Halo (out of 35
IVCs). Molecular hydrogen is seen in one high-velocity cloud (the Leading Arm
of the Magellanic Stream), although 19 high-velocity clouds are intersected;
this strongly suggests that dust is rare or absent in these objects. Finally,
there are five detections of H_2 in external galaxies.Comment: Accepted for ApJ Supplement. Note: figs 7 and 8 not included because
astro-ph rejects them as too bi
Determination of confusion noise for far-infrared measurements
We present a detailed assessment of the far-infrared confusion noise imposed
on measurements with the ISOPHOT far-infrared detectors and cameras aboard the
ISO satellite. We provide confusion noise values for all measurement
configurations and observing modes of ISOPHOT in the 90<=lambda<=200um
wavelength range. Based on these results we also give estimates for cirrus
confusion noise levels at the resolution limits of current and future
instruments of infrared space telescopes: Spitzer/MIPS, ASTRO-F/FIS and
Herschel/PACS.Comment: A&A accepted; FITS files and appendices are available at:
http://www.konkoly.hu/staff/pkisscs/confnoise
A search for interstellar bubbles surrounding massive stars in Perseus OB1
We have examined the interstellar medium in the vicinity of massive stars belonging to the Per OB1 association based on neutral hydrogen 21 cm observations obtained with the 100 m radio telescope at Effelsberg (HPBW = 8′.4) and complementary data from the Leiden-Dwingeloo H I Survey (HPBW = 36′). The higher angular resolution H I observations allowed us to discover probable wind-blown bubbles related to four massive stars in the association, namely, HD 14442 [O5n(f)p], HD 14947 [O5If+], HD 13022 [O9.5II-III((n))], and HD 13338 [O9.5V], while the detection of a wind-blown bubble associated with HD 16691 [O5If+] is less conclusive. A clear H I shell coincident in position with two B1III stars (HD 15233 and Hilt 311) was also detected. Some of these features also have infrared and/or molecular counterparts. The energetics of the structures related to each massive star is analyzed. The new H I interstellar bubbles appear to be similar to the ones found surrounding Wolf-Rayet stars and other Of stars. The large-scale maps obtained using the lower angular resolution H I data show that most of the early-type stars belonging to Per OB1 are placed in a region of low H I emission. The association could have blown a H I shell of about 350 Ă— 550 pc in size. This large H I shell has an infrared counterpart.Facultad de Ciencias AstronĂłmicas y GeofĂsicasInstituto Argentino de RadioastronomĂ
Power Spectrum Analysis of Far-IR Background Fluctuations in 160 Micron Maps From the Multiband Imaging Photometer for Spitzer
We describe data reduction and analysis of fluctuations in the cosmic far-IR
background (CFIB) in observations with the Multiband Imaging Photometer for
Spitzer (MIPS) instrument 160 micron detectors. We analyzed observations of an
8.5 square degree region in the Lockman Hole, part of the largest low-cirrus
mapping observation with this instrument. We measured the power spectrum of the
CFIB in these observations by fitting a power law to the IR cirrus component,
the dominant foreground contaminant, and subtracting this cirrus signal. The
CFIB power spectrum in the range 0.2 arc min^{-1} <k< 0.5 arc min^{-1} is
consistent with previous measurements of a relatively flat component. However,
we find a large power excess at low k, which falls steeply to the flat
component in the range 0.03 arc min^{-1} <k< 0.1 arc min^{-1}. This low-k power
spectrum excess is consistent with predictions of a source clustering
"signature". This is the first report of such a detection in the far-IR.Comment: This is the version of the paper accepted by A&A, which includes
various changes and new material. The superior-quality PDF with integrated
figures may be downloaded at
http://www-astro.lbl.gov/~bruce/spitzerpaper1/cfibaa_pub.pdf 15 pages,
figures integrated with text. This paper supersedes astro-ph/050416
The Spitzer Space Telescope First-Look Survey: Neutral Hydrogen Emission
The Spitzer Space Telescope (formerly SIRTF) extragalactic First-Look Survey
covered about 5 square degrees centered on J2000 17:18 +59:30 in order to
characterize the infrared sky with high sensitivity. We used the 100-m Green
Bank Telescope to image the 21cm Galactic HI emission over a 3x3 degree field
covering this position with an effective angular resolution of 9.8 arcmin and a
velocity resolution of 0.62 km/s. In the central square degree of the image the
average column density is N(HI) = 2.5 x 10^{20} cm-2 with an rms fluctuation of
0.3 x 10^{20}. The Galactic HI in this region has a very interesting structure.
There is a high-velocity cloud, several intermediate-velocity clouds (one of
which is probably part of the Draco nebula), and narrow-line low velocity
filaments. The HI emission shows a strong and detailed correlation with dust.
Except for the high-velocity cloud, all features in the HI map have
counterparts in an E(B-V) map derived from infrared data. Relatively high
E(B-V)/N(HI) ratios in some directions suggest the presence of molecular gas.
The best diagnostic of such regions is the peak HI line brightness temperature,
not the total N(HI): directions where Tb > 12 K have E(B-V)/N(HI) significantly
above the average value. The data corrected for stray radiation have been
released via the Web.Comment: Accepted for publication in the Astronomical Journal, April 2005. 25
pages includes 11 figures. The data and higher resolution figures are
available from http::/www.cv.nrao.edu/fls_gb
WHAM Observations of H-alpha Emission from High Velocity Clouds in the M, A, and C Complexes
The first observations of the recently completed Wisconsin H-Alpha Mapper
(WHAM) facility include a study of emission lines from high velocity clouds in
the M, A, and C complexes, with most of the observations on the M I cloud. We
present results including clear detections of H-alpha emission from all three
complexes with intensities ranging from 0.06 R to 0.20 R. In every observed
direction where there is significant high velocity H I gas seen in the 21 cm
line we have found associated ionized hydrogen emitting the H-alpha line. The
velocities of the H-alpha and 21 cm emission are well correlated in every case
except one, but the intensities are not correlated. There is some evidence that
the ionized gas producing the H-alpha emission envelopes the 21 cm emitting
neutral gas but the H-alpha "halo", if present, is not large. If the H-alpha
emission arises from the photoionization of the H I clouds, then the implied
Lyman continuum flux F_{LC} at the location of the clouds ranges from 1.3 to
4.2 x 10^5 photons cm^{-2} s^{-1}. If, on the other hand, the ionization is due
to a shock arising from the collision of the high-velocity gas with an ambient
medium in the halo, then the density of the pre-shocked gas can be constrained.
We have also detected the [S II] 6716 angstrom line from the M I cloud and have
evidence that the [S II] to H-alpha ratio varies with location on the cloud.Comment: 32 pages, 18 figures, to appear in ApJ (Sept. 10, 1998
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