42 research outputs found
Characterization of traverse slippage experienced by Spirit rover on Husband Hill at Gusev crater
Spirit rover experienced significant slips traversing Husband Hill. This paper analyzes the slippage Spirit experienced from Sol 154 to Sol 737. Slippage with respect to terrain type and slope is computed using data downlinked from the rover, rover position, and orientation estimations from visual odometry (VO) and photogrammetry based bundle adjustment (BA) method. Accumulated slippage reached a maximum of 83.86 m on Sol 648. However, as Spirit descended into the Inner Basin, the direction of slippage reversed, and accumulated slippage approached zero by the end of the entire traverse. Eight local regions with significant slips and nineteen traverse segments have been analyzed. Slippage was found to be highly correlated to slope direction and magnitude; the reverse of slope directions in the ascending and descending portions of the traverse proves to be the main contributor to the observed cancellation of slippage. While the horizontal component of the slippage almost canceled out, the difference in elevation continually accumulated, mainly during the ascent. In general, long traverse segments created more slips than short ones. This is reflected in both the accumulated and individual slippages. In considering the four major Mars terrain types, Spirit performed best on bedrock, managing to drive on slopes close to 30°. Fine-grain surfaces were the most challenging; though progress was made on slopes up to 15°, slippages of over 100% (more slippage than distance traveled) occurred for short segments. The results of this work can be incorporate into a traverse planning framework in which rover slippage is minimized. Results can be employed in landed planetary missions for precision navigation to avoid potentially dangerous regions by considering expected slippage
The RR Lyrae Distance Scale
We review seven methods of measuring the absolute magnitude M_V of RR Lyrae
stars in light of the Hipparcos mission and other recent developments. We focus
on identifying possible systematic errors and rank the methods by relative
immunity to such errors. For the three most robust methods, statistical
parallax, trigonometric parallax, and cluster kinematics, we find M_V (at
[Fe/H] = -1.6) of 0.77 +/- 0.13, 0.71 +/- 0.15, 0.67 +/- 0.10. These methods
cluster consistently around 0.71 +/- 0.07. We find that Baade-Wesselink and
theoretical models both yield a broad range of possible values (0.45-0.70 and
0.45-0.65) due to systematic uncertainties in the temperature scale and input
physics. Main-sequence fitting gives a much brighter M_V = 0.45 +/- 0.04 but
this may be due to a difference in the metallicity scales of the cluster giants
and the calibrating subdwarfs. White-dwarf cooling-sequence fitting gives 0.67
+/- 0.13 and is potentially very robust, but at present is too new to be fully
tested for systematics. If the three most robust methods are combined with
Walker's mean measurement for 6 LMC clusters, V_{0,LMC} = 18.98 +/- 0.03 at
[Fe/H] = -1.9, then mu_{LMC} = 18.33 +/- 0.08.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 21
pages including 1 table; uses Kluwer's crckapb.sty LaTeX style file, enclose
Distances and ages of NGC 6397, NGC 6752 and 47 Tuc
Distances and absolute ages for the globular clusters NGC6397, NGC6752, and
47 Tuc are obtained using Main Sequence Fitting. Accurate reddenings and metal
abundances (errors of 0.005 mag and 0.04 dex) are obtained by comparing B-V,
b-y colours and UVES spectra for clusters TO-stars and subgiants with similar
data for Hipparcos subdwarfs, using a reddening free temperature indicator
(Halpha). Errors in distances and ages are 0.07 mag and 1 Gyr. NGC6397 and
NGC6752 have ages of 13.5\pm 1.1 and 13.4\pm 1.1 Gyr, while 47 Tuc is probably
about 2.6 Gyr younger, in agreement with results obtained by techniques
sensitive to relative ages. Considering possible systematic errors, the age of
the oldest globular clusters is 13.4\pm 0.8\pm 0.6 Gyr (random and systematic
errors). This age is fully compatible with WMAP results, and indicates that the
oldest Galactic globular clusters formed within the first 1.7 Gyr after the Big
Bang, corresponding to a redshift of z\geq 2.5, in a standard LambdaCDM model.
The epoch of formation of the (inner halo) globular clusters lasted about 2.6
Gyr, ending at a time corresponding to a redshift of z\geq 1.3. Once combined
with values of Ho given by WMAP and by the HST Key Project, our age estimate
provides a robust upper limit at 95% level of confidence of Omega_M<0.57,
indicating the need for a vacuum energy independent of results from type Ia SNe
and clusters of galaxies. Using cluster distances, we derive a value of
M_V(HB)=(0.22\pm 0.05)([Fe/H]+1.5)+(0.56\pm 0.07) for the zero point of the
relation between the horizontal branch absolute magnitude and metallicity. When
combined with the apparent average luminosity of the RR Lyrae stars in the LMC
by Clementini et al. (2003), this zero point provides a new estimate of the
distance modulus to the LMC: (m-M)o=18.50\pm 0.09.Comment: 16 pages, 5 encapsulated figures, accepted for publication in A&
Multi-wavelength diagnostic properties of Galactic Planetary Nebulae detected by GLIMPSE-I
We uniformly analyze 136 optically detected PNe and candidates from the
GLIMPSE-I survey in order to to develop robust, multi-wavelength,
classification criteria to augment existing diagnostics and provide pure PN
samples. PNe represent powerful astrophysical probes. They are important
dynamical tracers, key sources of ISM chemical enrichment, windows into late
stellar evolution, and potent cosmological yardsticks. But their utility
depends on separating them unequivocally from the many nebular mimics which can
strongly resemble bona fide PNe in traditional optical images and spectra. We
merge new PNe from the carefully evaluated, homogeneous MASH-I and MASH-II
surveys, which offer a wider evolutionary range of PNe than hitherto available,
with previously known PNe classified by SIMBAD. Mid-infrared (MIR) measurements
vitally complement optical data because they reveal other physical processes
and morphologies via fine-structure lines, molecular bands and dust. MIR
colour-colour planes, optical emission line ratios and radio fluxes show the
unambiguous classification of PNe to be complex, requiring all available
evidence. Statistical trends provide predictive value and we offer quantitative
MIR criteria to determine whether an emission nebula is most likely to be a PN
or one of the frequent contaminants such as compact HII regions or symbiotic
systems. Prerequisites have been optical images and spectra but MIR morphology,
colours, environment and a candidate's MIR/radio flux ratio provide a more
rigorous classification. Our ultimate goal is to recognize PNe using only MIR
and radio characteristics, enabling us to trawl for PNe effectively even in
heavily obscured regions of the Galaxy.Comment: 32 pages, 18 figures, 10 table
Distances and ages of globular clusters using Hipparcos parallaxes of local subdwarfs
We discuss the impact of Population II and Globular Cluster (GCs) stars on
the derivation of the age of the Universe, and on the study of the formation
and early evolution of galaxies, our own in particular. The long-standing
problem of the actual distance scale to Population II stars and GCs is
addressed, and a variety of different methods commonly used to derive distances
to Population II stars are briefly reviewed. Emphasis is given to the
discussion of distances and ages for GCs derived using Hipparcos parallaxes of
local subdwarfs. Results obtained by different authors are slightly different,
depending on different assumptions about metallicity scale, reddenings, and
corrections for undetected binaries. These and other uncertainties present in
the method are discussed. Finally, we outline progress expected in the near
future.Comment: Invited review article to appear in: `Post-Hipparcos Cosmic Candles',
A. Heck & F. Caputo (Eds), Kluwer Academic Publ., Dordrecht, in press. 22
pages including 3 tables and 2 postscript figures, uses Kluwer's crckapb.sty
LaTeX style file, enclose
Observational Constraints on the Common Envelope Phase
The common envelope phase was first proposed more than forty years ago to
explain the origins of evolved, close binaries like cataclysmic variables. It
is now believed that the phase plays a critical role in the formation of a wide
variety of other phenomena ranging from type Ia supernovae through to binary
black holes, while common envelope mergers are likely responsible for a range
of enigmatic transients and supernova imposters. Yet, despite its clear
importance, the common envelope phase is still rather poorly understood. Here,
we outline some of the basic principles involved, the remaining questions as
well as some of the recent observational hints from common envelope phenomena -
namely planetary nebulae and luminous red novae - which may lead to answering
these open questions.Comment: 29 pages, 8 figures. To appear in the book "Reviews in Frontiers of
Modern Astrophysics: From Space Debris to Cosmology" (eds. Kabath, Jones and
Skarka; publisher Springer Nature) funded by the European Union Erasmus+
Strategic Partnership grant "Per Aspera Ad Astra Simul"
2017-1-CZ01-KA203-03556