94 research outputs found
Evidence for a rapid decrease in Pluto's atmospheric pressure revealed by a stellar occultation in 2019
We report observations of a stellar occultation by Pluto on 2019 July 17. A
single-chord high-speed (time resolution s) photometry dataset was
obtained with a CMOS camera mounted on the Tohoku University 60 cm telescope
(Haleakala, Hawaii). The occultation light curve is satisfactorily fitted to an
existing Pluto's atmospheric model. We find the lowest pressure value at a
reference radius of among those reported after 2012,
indicating a possible rapid (approximately of the previous
value) pressure drop between 2016 (the latest reported estimate) and 2019.
However, this drop is detected at a level only and still requires
confirmation from future observations. If real, this trend is opposite to the
monotonic increase of Pluto's atmospheric pressure reported by previous
studies. The observed decrease trend is possibly caused by ongoing
condensation processes in the Sputnik Planitia glacier associated with an
orbitally driven decline of solar insolation, as predicted by previous
theoretical models. However, the observed amplitude of the pressure decrease is
larger than the model predictions.Comment: 7 pages, 3 figures, accepted for publication in Astronomy and
Astrophysic
Vertical emissivity profiles of Jupiter's northern H-3(+) and H-2 infrared auroras observed by Subaru/IRCS
We resolved the vertical emissivity profiles of H-3(+) overtone, H-3(+) hot overtone, and H-2 emission lines of the Jovian northern auroras in K band obtained in December 2011 observed by the IR Camera and Spectrograph of the Subaru 8.2m telescope with the adaptive optics system (AO188). The spatial resolution achieved was similar to 0.2 arcsec, corresponding to similar to 600 km at Jupiter. We derived the vertical emissivity profiles at three polar regions close to the Jovian limb. The H-3(+) overtone and H-3(+) hot overtone lines had similar peak altitudes of 700-900 km and 680-950 km above the 1 bar level, which were 100-300 km and 150-420 km lower, respectively, than the model values. On the contrary, the H-2 peak emission altitude was high, 590-720 km above the 1 bar level. It was consistent with the value expected for precipitation of similar to 1 keV electron, which favors a higher-altitude emissivity profile. We concluded that the lower peak altitudes of H-3(+) overtone and hot overtone lines were caused by the nonlocal thermodynamic equilibrium effect stronger than the model assumption. We could reproduce the observational emissivity profiles from the model by including this effect. It has been proposed that neutral H-2 and ionized H-3(+) emissions can have different source altitudes because of their different morphologies and velocities; however, our observed results with a general circulation model show that the peak emission altitudes of H-3(+) and H-2 can be similar even with different velocities
High-precision polarimetry of nearby stars (d < 50 pc): Mapping the interstellar dust and magnetic field inside the Local Bubble
Context:We investigate the linear polarization produced by interstellar dust aligned by the magnetic field in the solar neighborhood (d Aims: We aim to detect and map dust clouds which give rise to statistically significant amounts of polarization of the starlight passing through the cloud, and to determine the interstellar magnetic field direction from the position angle of the observed polarization.Methods: High-precision broad-band (BV R) polarization observations are made of 361 stars in spectral classes F to G, with detection sensitivity at the level of or better than 10−5 (0.001%). The sample consists of 125 stars in the magnitude range 6–9 observed at the 2.2 m UH88 telescope on Mauna Kea, 205 stars in the magnitude range 3–6 observed at the Japanese (Tohoku) T60 telescope on Haleakala, and 31 stars in the magnitude range 4–7 observed at the 1.27 m H127 telescope of the Greenhill Observatory, Tasmania. Identical copies of the Dipol-2 polarimeter are used on these three sites.Results: Statistically significant (>3σ) polarization is found in 115 stars, and >2σ detection in 178 stars, out of the total sample of 361 stars. Polarization maps based on these data show filament-like patterns of polarization position angles, which are related to both the heliosphere geometry, the kinematics of nearby clouds, and the Interstellar Boundary EXplorer ribbon magnetic field. From long-term multiple observations, a number (~20) of stars show evidence of intrinsic variability at the 10−5 level. This can be attributed to circumstellar effects (e.g., debris disks and chromospheric activity). The star HD 101805 shows a peculiar wavelength dependence, indicating size distribution of scattering particles different from that of a typical interstellar medium. Our high signal-to-noise measurements of nearby stars with very low polarization also provide a useful dataset for calibration purposes
Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070
Aims. The optical emission of black hole transients increases by several
magnitudes during the X-ray outbursts. Whether the extra light arises from the
X-ray heated outer disc, from the inner hot accretion flow, or from the jet is
currently debated. Optical polarisation measurements are able to distinguish
the relative contributions of these components. Methods. We present the results
of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070
during the period of March-April 2018. Results. We detect small, 0.7%,
but statistically significant polarisation, part of which is of interstellar
origin. Depending on the interstellar polarisation estimate, the intrinsic
polarisation degree of the source is between 0.3% and 0.7%, and the
polarisation position angle is between . We show that the
polarisation increases after MJD 58222 (2018 April 14). The change is of the
order of 0.1% and is most pronounced in the R band. The change of the source
Stokes parameters occurs simultaneously with the drop of the observed V-band
flux and a slow softening of the X-ray spectrum. The Stokes vectors of
intrinsic polarisation before and after the drop are parallel, at least in the
V and R filters. Conclusions. We suggest that the increased polarisation is due
to the decreasing contribution of the non-polarized component, which we
associate with the the hot flow or jet emission. The low polarisation can
result from the tangled geometry of the magnetic field or from the Faraday
rotation in the dense, ionised, and magnetised medium close to the black hole.
The polarized optical emission is likely produced by the irradiated disc or by
scattering of its radiation in the optically thin outflow.Comment: 11 pages, 12 figures, A&A in pres
Orbital variability of the optical linear polarization of the -ray binary LS I +61 303 and new constraints on the orbital parameters
We studied the variability of the linear polarization and brightness of the
-ray binary LS I +61 303. High-precision BVR photopolarimetric
observations were carried out with the Dipol-2 polarimeter on the 2.2 m
remotely controlled UH88 telescope at Mauna Kea Observatory and the 60 cm
Tohoku telescope at Haleakala Observatory (Hawaii) over 140 nights in
2016--2019. We determined the position angle of the intrinsic polarization
, which can either be associated with the projection of
the Be star's decretion disk axis on the plane of sky, or can differ from it by
. Using the Lomb-Scargle method, we performed timing analyses and
period searches of our polarimetric and photometric data. We found
statistically significant periodic variability of the normalized Stokes
parameters and in all passbands. The most significant period of
variability, d, is equal to one half of the
orbital period d. Using a model of Thomson scattering
by a cloud that orbits the Be star, we obtained constraints on the orbital
parameters, including a small eccentricity and periastron phase of
, which coincides with the peaks in the radio, X-ray,
and TeV emission. These constraints are independent of the assumption about the
orientation of the decretion disk plane on the sky. We also extensively discuss
the apparent inconsistency with the previous measurements of the orbital
parameters from radial velocities. By folding the photometry data acquired
during a three-year time span with the orbital period, we found a linear phase
shift of the moments of the brightness maximum, confirming the possible
existence of superorbital variability.Comment: 15 pages, 16 figures, accepted for publication in A&
High-precision broad-band linear polarimetry of early-type binaries I. Discovery of variable, phase-locked polarization in HD48099
Aims. We investigate the structure of the O-type binary system HD48099 by measuring linear polarization that arises due to light scattering process. High-precison polarimetry provides independent estimates of the orbital parameters and gives important information on the properties of the system.Methods. Linear polarization measurements of HD48099 in the B, V and R passbands with the high-precision Dipol-2 polarimeter have been carried out. The data have been obtained with the 60 cm KVA (Observatory Roque de los Muchachos, La Palma, Spain) and T60 (Haleakala, Hawaii, USA) remotely controlled telescopes during 31 observing nights. Polarimetry in the optical wavelengths has been complemented by observations in the X-rays with the Swift space observatory.Results. Optical polarimetry revealed small intrinsic polarization in HD48099 with similar to 0.1% peak to peak variation over the orbital period of 3.08 d. The variability pattern is typical for binary systems, showing strong second harmonic of the orbital period. We apply our model code for the electron scattering in the circumstellar matter to put constraints on the system geometry. A good model fit is obtained for scattering of light on a cloud produced by the colliding stellar winds. The geometry of the cloud, with a broad distribution of scattering particles away from the orbital plane, helps in constraining the (low) orbital inclination. We derive from the polarization data the inclination i = 17 degrees +/- 2 degrees and the longitude of the ascending node Omega = 82 degrees +/- 1 degrees of the binary orbit. The available X-ray data provide additional evidence for the existence of the colliding stellar winds in the system. Another possible source of the polarized light could be scattering from the stellar photospheres. The models with circumstellar envelopes, or matter confined to the orbital plane, do not provide good constraints on the low inclination, better than i <= 27 degrees, as is already suggested by the absence of eclipses
High-precision polarimetry of nearby stars (d < 50 pc): Mapping the interstellar dust and magnetic field inside the Local Bubble
Context. We investigate the linear polarization produced by interstellar dust aligned by the magnetic field in the solar neighborhood (d Aims. We aim to detect and map dust clouds which give rise to statistically significant amounts of polarization of the starlight passing through the cloud, and to determine the interstellar magnetic field direction from the position angle of the observed polarization.Methods. High-precision broad-band (BV R) polarization observations are made of 361 stars in spectral classes F to G, with detection sensitivity at the level of or better than 10(-5) (0.001%). The sample consists of 125 stars in the magnitude range 6-9 observed at the 2.2 m UH88 telescope on Mauna Kea, 205 stars in the magnitude range 3-6 observed at the Japanese (Tohoku) T60 telescope on Haleakala, and 31 stars in the magnitude range 4-7 observed at the 1.27 m H127 telescope of the Greenhill Observatory, Tasmania. Identical copies of the Dipol-2 polarimeter are used on these three sites.Results. Statistically significant (>3 sigma) polarization is found in 115 stars, and >2 sigma detection in 178 stars, out of the total sample of 361 stars. Polarization maps based on these data show filament-like patterns of polarization position angles, which are related to both the heliosphere geometry, the kinematics of nearby clouds, and the Interstellar Boundary EXplorer ribbon magnetic field. From long-term multiple observations, a number (20) of stars show evidence of intrinsic variability at the 10(-5) level. This can be attributed to circumstellar effects (e.g., debris disks and chromospheric activity). The star HD 101805 shows a peculiar wavelength dependence, indicating size distribution of scattering particles different from that of a typical interstellar medium. Our high signal-to-noise measurements of nearby stars with very low polarization also provide a useful dataset for calibration purposes.</p
Orbital variability of the optical linear polarization of the γ -ray binary LS i +61° 303 and new constraints on the orbital parameters
We studied the variability of the linear polarization and brightness of the γ-ray binary LS I +61 303. High-precision BVR photopolarimetric observations were carried out with the Dipol-2 polarimeter on the 2.2 m remotely controlled UH88 telescope at Mauna Kea Observatory and the 60 cm Tohoku telescope at Haleakala Observatory (Hawaii) over 140 nights in 2016--2019. We determined the position angle of the intrinsic polarization θ≃11∘, which can either be associated with the projection of the Be star's decretion disk axis on the plane of sky, or can differ from it by 90∘. Using the Lomb-Scargle method, we performed timing analyses and period searches of our polarimetric and photometric data. We found statistically significant periodic variability of the normalized Stokes parameters q and u in all passbands. The most significant period of variability, PPol=13.244±0.012 d, is equal to one half of the orbital period Porb=26.496 d. Using a model of Thomson scattering by a cloud that orbits the Be star, we obtained constraints on the orbital parameters, including a small eccentricity e</p
Evolving optical polarisation of the black hole X-ray binary MAXI J1820+070
Aims. The optical emission of black hole transients increases by several magnitudes during the X-ray outbursts. Whether the extra light arises from the X-ray heated outer disc, from the inner hot accretion flow, or from the jet is currently debated. Optical polarisation measurements are able to distinguish the relative contributions of these components.Methods. We present the results of BVR polarisation measurements of the black hole X-ray binary MAXI J1820+070 during the period of March-April 2018.Results. We detect small, similar to 0.7%, but statistically significant polarisation, part of which is of interstellar origin. Depending on the interstellar polarisation estimate, the intrinsic polarisation degree of the source is between similar to 0.3% and 0.7%, and the polarisation position angle is between similar to 10 degrees-30 degrees. We show that the polarisation increases after MJD 58222 (2018 April 14). The change is of the order of 0.1% and is most pronounced in the R band. The change of the source Stokes parameters occurs simultaneously with the drop of the observed V-band flux and a slow softening of the X-ray spectrum. The Stokes vectors of intrinsic polarisation before and after the drop are parallel, at least in the V and R filters.Conclusions. We suggest that the increased polarisation is due to the decreasing contribution of the non-polarized component, which we associate with the the hot flow or jet emission. The low polarisation can result from the tangled geometry of the magnetic field or from the Faraday rotation in the dense, ionised, and magnetised medium close to the black hole. The polarized optical emission is likely produced by the irradiated disc or by scattering of its radiation in the optically thin outflow
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