718 research outputs found
The Production Rate and Employment of Ph.D. Astronomers
In an effort to encourage self-regulation of the astronomy job market, I
examine the supply of, and demand for, astronomers over time. On the supply
side, I document the production rate of Ph.D. astronomers from 1970 to 2006
using the UMI Dissertation Abstracts database, along with data from other
independent sources. I compare the long-term trends in Ph.D. production with
federal astronomy research funding over the same time period, and I demonstrate
that additional funding is correlated with higher subsequent Ph.D. production.
On the demand side, I monitor the changing patterns of employment using
statistics about the number and types of jobs advertised in the AAS Job
Register from 1984 to 2006. Finally, I assess the sustainability of the job
market by normalizing this demand by the annual Ph.D. production. The most
recent data suggest that there are now annual advertisements for about one
postdoctoral job, half a faculty job, and half a research/support position for
every new domestic Ph.D. recipient in astronomy and astrophysics. The average
new astronomer might expect to hold up to 3 jobs before finding a steady
position.Comment: 5 pages, 4 figures, 2 tables, PASP accepte
The abundance of HCN in circumstellar envelopes of AGB stars of different chemical types
A multi-transition survey of HCN (sub-) millimeter line emission from a large
sample of AGB stars of different chemical type is presented. The data are
analysed and circumstellar HCN abundances are estimated. The sample stars span
a large range of properties such as mass-loss rate and photospheric C/O-ratio.
The analysis of the new data allows for more accurate estimates of the
circumstellar HCN abundances and puts new constraints on chemical models. In
order to constrain the circumstellar HCN abundance distribution a detailed
non-LTE excitation analysis, based on the Monte Carlo method, is performed.
Effects of line overlaps and radiative excitation from dust grains are
included. The median values for the derived abundances of HCN (with respect to
H2) are 3x10-5, 7x10-7 and 10-7 for carbon stars (25 stars), S-type AGB stars
(19 stars) and M-type AGB stars (25 stars), respectively. The estimated sizes
of the HCN envelopes are similar to those obtained in the case of SiO for the
same sample of sources and agree well with previous results from
interferometric observations, when these are available. We find that there is a
clear dependence of the derived circumstellar HCN abundance on the C/O-ratio of
the star, in that carbon stars have about two orders of magnitude higher
abundances than M-type AGB stars, on average. The derived HCN abundances of the
S-type AGB stars have a larger spread and typically fall in between those of
the two other types, however, slightly closer to the values for the M-type AGB
stars. For the M-type stars, the estimated abundances are much higher than what
would be expected if HCN is formed in thermal equilibrium. However, the results
are also in contrast to predictions from recent non-LTE chemical models, where
very little difference is expected in the HCN abundances between the various
types of AGB stars.Comment: Accepted for publication in A&
Observational constraints on the effective climate sensitivity from the historical period
The observed warming in the atmosphere and ocean can be used to estimate the climate sensitivity linked to present-day feedbacks, which is referred to as the effective climate sensitivity (Shist ). However, such an estimate is affected by uncertainty in the radiative forcing, particularly aerosols, over the historical period. Here, we make use of detection and attribution techniques to derive the surface air temperature and ocean warming that can be attributed directly to greenhouse gas increases. These serve as inputs to a simple energy budget to infer the likelihood of Shist in response to observed greenhouse gases increases over two time periods (1862–2012 and 1955–2012). The benefit of using greenhouse gas attributable quantities is that they are not subject to uncertainties in the aerosol forcing (other than uncertainty in the attribution to greenhouse gas versus aerosol forcing not captured by the multi-model aerosol response pattern). The resulting effective climate sensitivity estimate, Shist , ranges from 1.3 °C to 3.1 °C (5%–95% range) over the full instrumental period (1862–2012) for our best estimate, and gets slightly wider when considering further uncertainties. This estimate increases to 1.7 °C–4.6 °C if using the shorter period (1955–2012). We also evaluate the climate model simulated surface air temperature and ocean heat content increase in response to greenhouse gas forcing over the same periods, and compare them with the observationally-constrained values. We find that that the ocean warming simulated in greenhouse gas only simulations in models considered here is consistent with that attributed to greenhouse gas increases from observations, while one model simulates more greenhouse gas-induced surface air warming than observed. However, other models with sensitivity outside our range show greenhouse gas warming that is consistent with that attributed in observations, emphasising that feedbacks during the historical period may differ from the feedbacks at CO2 doubling and from those at true equilibrium
HI observations of nearby galaxies I. The first list of the Karachentsev catalog
We present HI observations of the galaxies in the first list of the
Karachentsev catalog of previously unknown nearby dwarf galaxies (Karachentseva
& Karachentsev 1998). This survey covers all known nearby galaxy groups within
the Local Volume (i.e. within 10 Mpc) and their environment, that is about 25%
of the total sky. A total of 257 galaxies have been observed with a detection
rate of 60%. We searched a frequency band corresponding to heliocentric radial
velocities from -470 km/s to about +4000 km/s. Non-detections are either due to
limited coverage in radial velocity, confusion with Local HI (mainly in the
velocity range -140 km/s to +20 km/s, or lack of sensitivity for very weak
emission. 25% of the detected galaxies are located within the Local Volume.
Those galaxies are dwarf galaxies judged by their optical linear diameter (1.4
+/- 0.2 kpc on the average), their mean total HI mass (4.6 E7 solar masses),
and their observed linewidths (39 km/s).Comment: 22 pages, 9 ps figures, A&AS, in pres
The vacuum bubbles in de Sitter background and black hole pair creation
We study the possible types of the nucleation of vacuum bubbles. We classify
vacuum bubbles in de Sitter background and present some numerical solutions.
The thin-wall approximation is employed to obtain the nucleation rate and the
radius of vacuum bubbles. With careful analysis we confirm that Parke's formula
is also applicable to the large true vacuum bubbles. The nucleation of the
false vacuum bubble in de Sitter background is also evaluated. The tunneling
process in the potential with degenerate vacua is analyzed as the limiting
cases of the large true vacuum bubble and false vacuum bubble. Next, we
consider the pair creation of black holes in the background of bubble
solutions. We obtain static bubble wall solutions of junction equation with
black hole pair. The masses of created black holes are uniquely determined by
the cosmological constant and surface tension on the wall. Finally, we obtain
the rate of pair creation of black holes.Comment: 3 figures, minor including errors and typos corrected, and refs.
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Explaining Earths Energy Budget: CERES-Based NASA Resources for K-12 Education and Public Outreach
Among atmospheric scientists, the importance of the Earth radiation budget concept is well understood. Papers have addressed the topic for over 100 years, and the large Clouds and the Earth's Radiant Energy System (CERES) science team (among others), with its multiple on-orbit instruments, is working hard to quantify the details of its various parts. In education, Earth's energy budget is a concept that generally appears in middle school and Earth science curricula, but its treatment in textbooks leaves much to be desired. Students and the public hold many misconceptions, and very few people have an appreciation for the importance of this energy balance to the conditions on Earth. More importantly, few have a correct mental model that allows them to make predictions and understand the effect of changes such as increasing greenhouse gas concentrations. As an outreach element of the core CERES team at NASA Langley, a multi-disciplinary group of scientists, educators, graphic artists, writers, and web developers has been developing and refining graphics and resources to explain the Earth's Energy budget over the last few decades. Resources have developed through an iterative process involving ongoing use in front of a variety of audiences, including students and teachers from 3rd to 12th grade as well as public audiences
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