475 research outputs found
Short-term variability of comet C/2012 S1 (ISON) at 4.8 AU from the Sun
We observed comet C/2012 S1 (ISON) during six nights in February 2013 when it
was at 4.8 AU from the sun. At this distance and time the comet was not very
active and it was theoretically possible to detect photometric variations
likely due to the rotation of the cometary nucleus. The goal of this work is to
obtain differential photometry of the comet inner coma using different aperture
radii in order to derive a possible rotational period. Large field of view
images were obtained with a 4k x 4k CCD at the f/3 0.77m telescope of La Hita
Observatory in Spain. Aperture photometry was performed in order to get
relative magnitude variation versus time. Using calibrated star fields we also
obtained ISON's R-magnitudes versus time. We applied a Lomb-Scargle periodogram
analysis to get possible periodicities for the observed brightness variations,
directly related with the rotation of the cometary nucleus. The comet light
curve obtained is very shallow, with a peak-to-peak amplitude of 0.03
0.02 mag. A tentative synodic rotational period (single-peaked) of 14.4
1.2 hours for ISON's nucleus is obtained from our analysis, but there are other
possibilities. We studied the possible effect of the seeing variations in the
obtained periodicities during the same night, and from night to night. These
seeing variations had no effect on the derived periodicity. We discuss and
interpret all possible solutions for the rotational period of ISON's nucleus.Comment: 15 pages, 3 Figures, 2 Tables, accepted for publication in A&
Activity of (2060) Chiron possibly caused by impacts?
The centaur 95P/(2060) Chiron is showing comet-like activity since its
discovery, but the mass-loss mechanisms triggering its activity remained
unexplained. Although the collision rates in the centaur region are expected to
be very low, and impacts are thought not to be responsible for the mass-loss,
since the recent indications that Chiron might possess a ring similar to
Chariklo's, and assuming that there is debris orbiting around, the impact
triggered mass-loss mechanism should not be excluded as a possible cause of its
activity. From time series observations collected on Calar Alto Observatory in
Spain between 2014 and 2016, we found that the photometric scatter in Chiron's
data is larger than a control star's scatter, indicating a possible
microactivity, possibly caused by debris falling back to Chiron's surface and
lifting small clouds of material. We also present rotational light curves, and
measurements of Chiron's absolute magnitudes, that are consistent with the
models supporting the presumption that Chiron possesses rings. By co-adding the
images acquired in 2015, we have detected a 5 arcsec long tail, showing
a surface brightness of 25.3 mag(V)/arcsec.Comment: Accepted for publication in Monthly Notices of the Royal Astronomical
Society (MNRAS) on 2017 December 2
A mid-term astrometric and photometric study of Trans-Neptunian Object (90482) Orcus
From CCD observations of a fixed and large star field that contained the
binary TNO Orcus, we have been able to derive high-precision relative
astrometry and photometry of the Orcus system with respect to background stars.
The RA residuals of an orbital fit to the astrometric data revealed a
periodicity of 9.7+-0.3 days, which is what one would expect to be induced by
the known Orcus companion. The residuals are also correlated with the
theoretical positions of the satellite with regard to the primary. We therefore
have revealed the presence of Orcus' satellite in our astrometric measurements.
The photocenter motion is much larger than the motion of Orcus around the
barycenter, and we show here that detecting some binaries through a carefully
devised astrometric technique might be feasible with telescopes of moderate
size. We also analyzed the system's mid-term photometry to determine whether
the rotation could be tidally locked to the satellite's orbital period. We
found that a photometric variability of 9.7+-0.3 days is clear in our data, and
is nearly coincident with the orbital period of the satellite. We believe this
variability might be induced by the satellite's rotation. There is also a
slight hint for an additional small variability in the 10 hr range that was
already reported in the literature. This short-term variability would indicate
that the primary is not tidally locked and therefore the system would not have
reached a double synchronous state. Implications for the basic physical
properties of the primary and its satellite are discussed. From angular
momentum considerations we suspect that the Orcus satellite might have formed
from a rotational fission. This requires that the mass of the satellite would
be around 0.09 times that of the primary, close to the value that one derives
by using an albedo of 0.12 for the satellite and assuming equal densities for
both objects.Comment: in Press at A&
The albedo-color diversity of transneptunian objects
We analyze albedo data obtained using the Herschel Space Observatory that
reveal the existence of two distinct types of surface among midsized
transneptunian objects. A color-albedo diagram shows two large clusters of
objects, one redder and higher albedo and another darker and more neutrally
colored. Crucially, all objects in our sample located in dynamically stable
orbits within the classical Kuiper belt region and beyond are confined to the
bright-red group, implying a compositional link. Those objects are believed to
have formed further from the Sun than the dark-neutral bodies. This
color-albedo separation is evidence for a compositional discontinuity in the
young solar system.Comment: 16 pages, 4 figures, 1 table, published in ApJL (12 August 2014), The
Astrophysical Journal (2014), vol. 793, L
Transneptunian objects and Centaurs from light curves
We analyze a vast light curve database by obtaining mean rotational
properties of the entire sample, determining the spin frequency distribution
and comparing those data with a simple model based on hydrostatic equilibrium.
For the rotation periods, the mean value obtained is 6.95 h for the whole
sample, 6.88 h for the Trans-neptunian objects (TNOs) alone and 6.75 h for the
Centaurs. From Maxwellian fits to the rotational frequencies distribution the
mean rotation rates are 7.35 h for the entire sample, 7.71 h for the TNOs alone
and 8.95 h for the Centaurs. These results are obtained by taking into account
the criteria of considering a single-peak light curve for objects with
amplitudes lower than 0.15 mag and a double-peak light curve for objects with
variability >0.15mag. The best Maxwellian fits were obtained with the threshold
between 0.10 and 0.15mag. The mean light-curve amplitude for the entire sample
is 0.26 mag, 0.25mag for TNOs only, and 0.26mag for the Centaurs. The amplitude
versus Hv correlation clearly indicates that the smaller (and collisionally
evolved) objects are more elongated than the bigger ones. From the model
results, it appears that hydrostatic equilibrium can explain the statistical
results of almost the entire sample, which means hydrostatic equilibrium is
probably reached by almost all TNOs in the H range [-1,7]. This implies that
for plausible albedos of 0.04 to 0.20, objects with diameters from 300km to
even 100km would likely be in equilibrium. Thus, the great majority of objects
would qualify as being dwarf planets because they would meet the hydrostatic
equilibrium condition. The best model density corresponds to 1100 kg/m3.Comment: 21 pages, 8 figures. Astronomy & Astrophysics, in pres
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