1,559 research outputs found
Advanced mission analysis programs
Computer programs provide preliminary trajectory and guidance information required for feasibility studies in space mission analysis. The advanced mission analysis computer programs include programs for approximate solutions, programs for targeting and output, and programs for Monte Carlo and linear guidance analysis
Diagnostics Of Disks Around Hot Stars
We discuss three different observational diagnostics related to disks around hot stars: absorption line determinations of rotational velocities of Be stars; polarization diagnostics of circumstellar disks; and X-ray line diagnostics of one specific magnetized hot star, theta(1) Ori C. Some common themes that emerge from these studies include (a) the benefits of having a specific physical model as a framework for interpreting diagnostic data; (b) the importance of combining several different types of observational diagnostics of the same objects; and (c) that while there is often the need to reinterpret traditional diagnostics in light of new theoretical advances, there are many new and powerful diagnostics that are, or will soon be, available for the study of disks around hot stars
Anomalous CO2 Ice Toward HOPS-68: A Tracer of Protostellar Feedback
We report the detection of a unique CO2 ice band toward the deeply embedded,
low-mass protostar HOPS-68. Our spectrum, obtained with the Infrared
Spectrograph onboard the Spitzer Space Telescope, reveals a 15.2 micron CO2 ice
bending mode profile that cannot modeled with the same ice structure typically
found toward other protostars. We develop a modified CO2 ice profile
decomposition, including the addition of new high-quality laboratory spectra of
pure, crystalline CO2 ice. Using this model, we find that 87-92% of the CO2 is
sequestered as spherical, CO2-rich mantles, while typical interstellar ices
show evidence of irregularly-shaped, hydrogen-rich mantles. We propose that (1)
the nearly complete absence of unprocessed ices along the line-of-sight is due
to the flattened envelope structure of HOPS-68, which lacks cold absorbing
material in its outer envelope, and possesses an extreme concentration of
material within its inner (10 AU) envelope region and (2) an energetic event
led to the evaporation of inner envelope ices, followed by cooling and
re-condensation, explaining the sequestration of spherical, CO2 ice mantles in
a hydrogen-poor mixture. The mechanism responsible for the sublimation could be
either a transient accretion event or shocks in the interaction region between
the protostellar outflow and envelope. The proposed scenario is consistent with
the rarity of the observed CO2 ice profile, the formation of nearly pure CO2
ice, and the production of spherical ice mantles. HOPS-68 may therefore provide
a unique window into the protostellar feedback process, as outflows and heating
shape the physical and chemical structure of protostellar envelopes and
molecular clouds.Comment: Accepted to the Astrophysical Journal, 2013 February 15: 14 pages, 9
figures, 3 table
Evolution of the Inner Circumstellar Envelope of V838 Monocerotis
We present imaging polarimetry observations of the eruptive variable V838
Monocerotis and its neighboring field obtained in 2002 October. The
polarization of field stars confirms the previously determined interstellar
polarization along the line of sight to V838 Mon. While V838 Mon showed
intrinsic polarization shortly after its second outburst on 2002 February 8,
all subsequent observations only showed a quiescent interstellar polarization
component. We find V838 Mon once again showed significant intrinsic
polarization in 2002 October, suggesting the presence of an asymmetrical
geometry of scattering material close to the star. Furthermore, an observed 90
degree position angle flip in the intrinsic polarization from 2002 February to
2002 October suggests that the distribution of nearby circumstellar material
has experienced significant changes. We discuss the opacity changes in the
evolving circumstellar cloud around V838 Mon that may explain these
observations.Comment: accepted by ApJL, 11 pages (including 2 figs, 2 tables
TRENDS: A flight test relational database user's guide and reference manual
This report is designed to be a user's guide and reference manual for users intending to access rotocraft test data via TRENDS, the relational database system which was developed as a tool for the aeronautical engineer with no programming background. This report has been written to assist novice and experienced TRENDS users. TRENDS is a complete system for retrieving, searching, and analyzing both numerical and narrative data, and for displaying time history and statistical data in graphical and numerical formats. This manual provides a 'guided tour' and a 'user's guide' for the new and intermediate-skilled users. Examples for the use of each menu item within TRENDS is provided in the Menu Reference section of the manual, including full coverage for TIMEHIST, one of the key tools. This manual is written around the XV-15 Tilt Rotor database, but does include an appendix on the UH-60 Blackhawk database. This user's guide and reference manual establishes a referrable source for the research community and augments NASA TM-101025, TRENDS: The Aeronautical Post-Test, Database Management System, Jan. 1990, written by the same authors
Achernar: Rapid Polarization Variability as Evidence of Photospheric and Circumstellar Activity
We present the results of a high accuracy ()
polarization monitoring of the Be Star Achernar that was carried out between
July 7th and November 5th, 2006. Our results indicate that, after a near
quiescent phase from 1998 to 2002, Achernar is presently in an active phase and
has built a circumstellar disk. We detect variations both in the polarization
level and position angle in timescales as short as one hour and as long as
several weeks. Detailed modeling of the observed polarization strongly suggests
that the short-term variations originate from discrete mass ejection events
which produce transient inhomogeneities in the inner disk. Long-term
variations, on the other hand, can be explained by the formation of an inner
ring following one or several mass ejection events.Comment: 16 pages, 5 figures, Accepted to Ap
Lem2p (LEM2) and Cmp7p (CHMP7) function in ESCRT-dependent nuclear envelope remodeling in fission yeast
ESCRTâIII proteins have been implicated in sealing the nuclear envelope in mammals, both during nuclear assembly and following mechanical disruption. This sealing process requires the ESCRTâII/ESCRTâ III hybrid protein CHMP7 and the AAA ATPase VPS4. It remains unclear, however, how
CHMP7 is recruited to breaches of the nuclear envelope. The fission yeast S. pombe is an attractive genetic model system for investigating this role of the ESCRT pathway because, in fission yeast, the nuclear envelope develops fenestrations that must be closed twice per cell cycle: upon mitotic entry when duplicated spindle pole bodies (SPB) are incorporated into the nuclear envelope and
after a successful cell cycle when the SPBs are ejected back to cytoplasm. Here we report that deletion of fission yeast vps4 leads to severe defects in nuclear morphology and integrity, which causes delayed segregation of duplicated SPBs, asymmetric nuclear bipartition in mitosis, and slow growth. Interestingly, these phenotypes are spontaneously suppressed by lossâofâfunction mutations
that arise in cmp7 (pombe CHMP7) or lem2, a member of the LEM (Lap2âEmerinâMan1) family of inner nuclear membrane proteinsâimplying that all three function in the same pathway. Based on these observations, we hypothesize that Lem2p acts as a nuclear siteâspecific adaptor to recruit Cmp7p to the nuclear envelope
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