899 research outputs found
The Fine-Scale Structure of the neutral Interstellar Medium in nearby Galaxies
We present an analysis of the properties of HI holes detected in 20 galaxies
that are part of "The HI Nearby Galaxy Survey" (THINGS). We detected more than
1000 holes in total in the sampled galaxies. Where they can be measured, their
sizes range from about 100 pc (our resolution limit) to about 2 kpc, their
expansion velocities range from 4 to 36 km/s, and their ages are estimated to
range between 3 and 150 Myr. The holes are found throughout the disks of the
galaxies, out to the edge of the HI; 23% of the holes fall outside R25. We find
that shear limits the age of holes in spirals (shear is less important in dwarf
galaxies) which explains why HI holes in dwarfs are rounder, on average than in
spirals. Shear, which is particularly strong in the inner part of spiral
galaxies, also explains why we find that holes outside R25 are larger and
older. We derive the scale height of the HI disk as a function of
galactocentric radius and find that the disk flares up in all galaxies. We
proceed to derive the surface and volume porosity (Q2D and Q3D) and find that
this correlates with the type of the host galaxy: later Hubble types tend to be
more porous. The size distribution of the holes in our sample follows a power
law with a slope of a ~ -2.9. Assuming that the holes are the result of massive
star formation, we derive values for the supernova rate (SNR) and star
formation rate (SFR) which scales with the SFR derived based on other tracers.
If we extrapolate the observed number of holes to include those that fall below
our resolution limit, down to holes created by a single supernova, we find that
our results are compatible with the hypothesis that HI holes result from star
formation.Comment: 142 pages, 55 figures, accepted for publication in the Astronomical
Journa
IGR J17254-3257, a new bursting neutron star
The study of the observational properties of uncommonly long bursts from low
luminosity sources with extended decay times up to several tens of minutes is
important when investigating the transition from a hydrogen-rich bursting
regime to a pure helium regime and from helium burning to carbon burning as
predicted by current burst theories. IGR J17254-3257 is a recently discovered
X-ray burster of which only two bursts have been recorded: an ordinary short
type I X-ray burst, and a 15 min long burst. An upper limit to its distance is
estimated to about 14.5 kpc. The broad-band spectrum of the persistent emission
in the 0.3-100 keV energy band obtained using contemporaneous INTEGRAL and
XMM-Newton data indicates a bolometric flux of 1.1x10^-10 erg/cm2/s
corresponding, at the canonical distance of 8 kpc, to a luminosity about
8.4x10^35 erg/s between 0.1-100 keV, which translates to a mean accretion rate
of about 7x10^-11 solar masses per year. The low X-ray persistent luminosity of
IGR J17254-3257 seems to indicate the source may be in a state of low accretion
rate usually associated with a hard spectrum in the X-ray range. The nuclear
burning regime may be intermediate between pure He and mixed H/He burning. The
long burst is the result of the accumulation of a thick He layer, while the
short one is a prematurate H-triggered He burning burst at a slightly lower
accretion rate.Comment: 4 pages, 4 figures, 1 table; accepted for publication in A&A Letters.
1 reference (Cooper & Narayan, 2007) correcte
American Etiquette and Rules of Politeness (Part One)
This is a volume dedicated to explaining American etiquette, which the authors consider to be, The most complete work on Etiquette that has yet been presented to the public. This first part of the book covers the value of etiquette, good manners, social intercourse, home etiquette, home culture, entrance into society, introductions, salutations, conversation, table etiquette, street etiquette, traveling, riding, driving, public etiquette, calling and visiting, receptions, parties, dinners, women\u27s higher culture, courtship, marriage, wedding etiquette, good conduct, anniversaries, personal care and hygiene, and clothing.https://openworks.wooster.edu/motherhomeheaven/1080/thumbnail.jp
American Etiquette and Rules of Politeness (Part Two)
This is a volume dedicated to explaining American etiquette, which the authors consider to be, The most complete work on Etiquette that has yet been presented to the public. This second part of the book discusses the etiquette of gifts, business, letter writing, notes, cards, funerals, addressing foreign people with titles, games, and amusements. It also discusses specific etiquette for Washington, D.C. The final chapters explain the language of flowers, the significance of precious stones, and recipes for personal care.https://openworks.wooster.edu/motherhomeheaven/1081/thumbnail.jp
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Gas distribution, kinematics and star formation in faint dwarf galaxies
We compare the gas distribution, kinematics and the current star formation in
a sample of 10 very faint (-13.37 < M_B < -9.55) dwarf galaxies. For 5 of these
galaxies we present fresh, high sensitivity, GMRT HI 21cm observations. For all
our galaxies we construct maps of the HI column density at a constant linear
resolution of ~300 pc; this forms an excellent data set to check for the
presence of a threshold column density for star formation. We find that while
current star formation (as traced by Halpha emission) is confined to regions
with relatively large (N_HI > (0.4 -1.7) X 10^{21} atoms cm^{-2}) HI column
density, the morphology of the Halpha emission is in general not correlated
with that of the high HI column density gas. Thus, while high column density
gas may be necessary for star formation, in this sample at least, it is not
sufficient to ensure that star formation does in fact occur. We examine the
line profiles of the HI emission, but do not find a simple relation between
regions with complex line profiles and those with on-going star formation.
Finally, we examine the very fine scale (~20-100 pc) distribution of the HI
gas, and find that at these scales the emission exhibits a variety of shell
like, clumpy and filamentary features. The Halpha emission is sometimes
associated with high density HI clumps, sometimes the Halpha emission lies
inside a high density shell, and sometimes there is no correspondence between
the Halpha emission and the HI clumps. In summary, the interplay between star
formation and gas density in these galaxy does not seem to show the simple
large scale patterns observed in brighter galaxies (abridged).Comment: 15 pages, 6 tables, 13 figures. Accepted for publication in MNRA
Chemostratigraphy of Neoproterozoic carbonates: implications for 'blind dating'
The delta C-13(carb) and Sr-87/Sr-86 secular variations in Neoproteozoic seawater have been used for the purpose of 'isotope stratigraphy' but there are a number of problems that can preclude its routine use. In particular, it cannot be used with confidence for 'blind dating'. The compilation of isotopic data on carbonate rocks reveals a high level of inconsistency between various carbon isotope age curves constructed for Neoproteozoic seawater, caused by a relatively high frequency of both global and local delta C-13(carb) fluctuations combined with few reliable age determinations. Further complication is caused by the unresolved problem as to whether two or four glaciations, and associated negative delta C-13(carb) excursions, can be reliably documented. Carbon isotope stratigraphy cannot be used alone for geological correlation and 'blind dating'. Strontium isotope stratigraphy is a more reliable and precise tool for stratigraphic correlations and indirect age determinations. Combining strontium and carbon isotope stratigraphy, several discrete ages within the 590-544 Myr interval, and two age-groups at 660-610 and 740-690 Myr can be resolved
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