232 research outputs found
Distances to the high galactic latitude molecular clouds G192-67 and MBM 23-24
We report on distance determinations for two high Galactic latitude cloud
complexes, G192-67 and MBM 23-24. No distance determination exists in the
literature for either cloud. Thirty-four early type stars were observed towards
the two clouds, more than half of which have parallaxes measured by the
Hipparcos satellite. For the remaining stars we have made spectroscopic
distance estimates. The data consist of high resolution echelle spectra
centered on the Na I D lines, and were obtained over six nights at the Coude
Feed telescope at Kitt Peak National Observatory. Interstellar absorption lines
were detected towards some of the stars, enabling estimates of the distances to
the clouds of 109 +/- 14 pc for G192-67, and of 139 +/- 33 pc for MBM 23-24. We
discuss the relationship of these clouds to other ISM features such as the
Local Hot Bubble and the local cavity in neutral hydrogen.Comment: 15 pages, 6 embedded figures, to be published in the ApJ Vol. 516,
No.
Radar observations of geomagnetic disturbance effects on midlatitude mesosphere/lower thermosphere dynamics
Zeitreihen von Monatsmittelwerten des Windes in der Mesosphäre/unteren Thermosphäre über Collm werden auf mögliche Korrelationen mit der Nordatlantischen Oszillation (NAO) und der Südlichen Oszillation (SO) hin untersucht. Während eine positive Korrelation bis in die 1990er Jahre existiert, schwächt sich diese in der Folge ab und kehrt sich teilweise um. Da NAO und SO gekoppelt sind, erfolgen diese Änderungen etwa zur selben Zeit. Die Änderung der Kopplung steht wahrscheinlich in Verbindung mit einer generellen Änderung der Dynamik der mittleren Atmosphäre
Experimental analysis of gas-sensitive Braitenberg vehicles
This article addresses the problem of localizing a static gas source in an indoor environment by a mobile robot. In contrast to previous works, the environment is not artificially ventilated to produce a strong unidirectional airflow. Here, the dominant transport mechanisms of gas molecules are turbulence and convection flow rather than diffusion, which results in a patchy, chaotically fluctuating gas distribution. Two Braitenberg-type strategies (positive and negative tropotaxis) based on the instantaneously measured spatial concentration gradient were investigated. Both strategies were shown to be of potential use for gas source localization. As a possible solution to the problem of gas source declaration (the task of determining with certainty that the gas source has been found), an indirect localization strategy based on exploration and concentration peak avoidance is suggested. Here, a gas source is located by exploiting the fact that local concentration maxima occur more frequently near the gas source compared to distant regions
Bostonia: The Boston University Alumni Magazine. Volume 25
Founded in 1900, Bostonia magazine is Boston University's main alumni publication, which covers alumni and student life, as well as university activities, events, and programs
HI spectra and column densities toward HVC and IVC probes
We show 21-cm line profiles in the direction of stars and extragalactic
objects, lying projected on high- and intermediate-velocity clouds (HVCs and
IVCs). About half of these are from new data obtained with the Effelsberg 100-m
telescope, about a quarter are extracted from the Leiden-Dwingeloo Survey (LDS)
and the remaining quarter were observed with other single-dish telescopes. HI
column densities were determined for each HVC/IVC. Wakker (2001) (Paper I) uses
these in combination with optical and ultraviolet high-resolution measurements
to derive abundances. Here, an analysis is given of the difference and ratio of
N(HI) as observed with a 9 arcmin versus a 35 arcmin beam. For HVCs and IVCs
the ratio N(HI-9 arcmin)/N(HI-35 arcmin) lies in the range 0.2-2.5. For
low-velocity gas this ratio ranges from 0.75 to 1.3 (the observed ratio is
0.85-1.4, but it appears that the correction for stray radiation is slightly
off). The smaller range for the low-velocity gas may be caused by confusion in
the line of sight, so that a low ratio in one component can be compensated by a
high ratio in another -- for 11 low-velocity clouds fit by one component the
distribution of ratios has a larger dispersion. Comparison with higher angular
resolution data is possible for sixteen sightlines. Eight sightlines with HI
data at 1 arcmin-2 arcmin resolution show a range of 0.75-1.25 for N(HI-2
arcmin)/N(HI-9 arcmin), while in eight other sightlines N(HI-Ly-alpha)/N(HI-9
arcmin) ranges from 0.74 to 0.98.Comment: To appear in the "Astrophysical Journal Supplement"; 45 pages;
degraded figures (astro-ph restriction) - ask for good version
Smc5/6 coordinates formation and resolution of joint molecules with chromosome morphology to ensure meiotic divisions
During meiosis, Structural Maintenance of Chromosome (SMC) complexes underpin two fundamental features of meiosis: homologous recombination and chromosome segregation. While meiotic functions of the cohesin and condensin complexes have been delineated, the role of the third SMC complex, Smc5/6, remains enigmatic. Here we identify specific, essential meiotic functions for the Smc5/6 complex in homologous recombination and the regulation of cohesin. We show that Smc5/6 is enriched at centromeres and cohesin-association sites where it regulates sister-chromatid cohesion and the timely removal of cohesin from chromosomal arms, respectively. Smc5/6 also localizes to recombination hotspots, where it promotes normal formation and resolution of a subset of joint-molecule intermediates. In this regard, Smc5/6 functions independently of the major crossover pathway defined by the MutLγ complex. Furthermore, we show that Smc5/6 is required for stable chromosomal localization of the XPF-family endonuclease, Mus81-Mms4Eme1. Our data suggest that the Smc5/6 complex is required for specific recombination and chromosomal processes throughout meiosis and that in its absence, attempts at cell division with unresolved joint molecules and residual cohesin lead to severe recombination-induced meiotic catastroph
Distances and Metallicities of High- and Intermediate-Velocity Clouds
A table is presented that summarizes published absorption line measurements
for the high- and intermediate velocity clouds (HVCs and IVCs). New values are
derived for N(HI) in the direction of observed probes, in order to arrive at
reliable abundances and abundance limits (the HI data are described in Paper
II). Distances to stellar probes are revisited and calculated consistently, in
order to derive distance brackets or limits for many of the clouds, taking care
to properly interpret non-detections. The main conclusions are the following.
1) Absolute abundances have been measured using lines of SII, NI and OI, with
the following resulting values: ~0.1 solar for one HVC (complex C), ~0.3 solar
for the Magellanic Stream, ~0.5 solar for a southern IVC, and ~ solar for two
northern IVCs (the IV Arch and LLIV Arch). Finally, approximate values in the
range 0.5-2 solar are found for three more IVCs. 2) Depletion patterns in IVCs
are like those in warm disk or halo gas. 3) Most distance limits are based on
strong UV lines of CII, SiII and MgII, a few on CaII. Distance limits for major
HVCs are >5 kpc, while distance brackets for several IVCs are in the range
0.5-2 kpc. 4) Mass limits for major IVCs are 0.5-8x10^5 M_sun, but for major
HVCs they are >10^6 M_sun. 5) The CaII/HI ratio varies by up to a factor 2-5
within a single cloud, somewhat more between clouds. 6) The NaIHI ratio varies
by a factor >10 within a cloud, and even more between clouds. Thus, CaII can be
useful for determining both lower and upper distance limits, but NaI only
yields upper limits.Comment: To appear in the "Astrophysical Journal Supplement"; 82 pages;
figures 6, 9 and 10 are in color; degraded figures (astro-ph restriction) -
ask for good version
Specialized interfaces of Smc5/6 control hinge stability and DNA association
The Structural Maintenance of Chromosomes (SMC) complexes: cohesin, condensin and Smc5/6 are involved in the organization of higher-order chromosome structure—which is essential for accurate chromosome duplication and segregation. Each complex is scaffolded by a specific SMC protein dimer (heterodimer in eukaryotes) held together via their hinge domains. Here we show that the Smc5/6-hinge, like those of cohesin and condensin, also forms a toroidal structure but with distinctive subunit interfaces absent from the other SMC complexes; an unusual ‘molecular latch’ and a functional ‘hub’. Defined mutations in these interfaces cause severe phenotypic effects with sensitivity to DNA-damaging agents in fission yeast and reduced viability in human cells. We show that the Smc5/6-hinge complex binds preferentially to ssDNA and that this interaction is affected by both ‘latch’ and ‘hub’ mutations, suggesting a key role for these unique features in controlling DNA association by the Smc5/6 complex
- …