Disk-Integrated Thermal Properties of Ceres Measured at Millimeter Wavelengths. (arXiv:2003.11045v1 [astro-ph.EP])

Disk-Integrated Thermal Properties of Ceres Measured at Millimeter Wavelengths. (arXiv:2003.11045v1 [astro-ph.EP])
<a href="http://arxiv.org/find/astro-ph/1/au:+Li_J/0/1/0/all/0/1">Jian-Yang Li</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Moullet_A/0/1/0/all/0/1">Arielle Moullet</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Titus_T/0/1/0/all/0/1">Timothy N. Titus</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Hsieh_H/0/1/0/all/0/1">Henry H. Hsieh</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Sykes_M/0/1/0/all/0/1">Mark V. Sykes</a>

We observed Ceres at three epochs in 2015 November and 2017 September and
October with ALMA 12-meter array and in 2017 October with the ALMA Compact
Array (ACA), all at ~265 GHz continuum (wavelengths of ~1.1 mm) to map the
temperatures of Ceres over a full rotation at each epoch. We also used 2017
October ACA observations to search for HCN. The disk-averaged brightness
temperature of Ceres is measured to be between 170 K and 180 K during our 2017
observations. The rotational lightcurve of Ceres shows a double peaked shape
with an amplitude of about 4%. Our HCN search returns a negative result with an
upper limit production rate of ~2$times$10$^{24}$ molecules s$^{-1}$, assuming
globally uniform production and a Haser model. A thermophysical model suggests
that Ceres’s top layer has higher dielectric absorption than lunar-like
materials at a wavelength of 1 mm. However, previous observations showed that
the dielectric absorption of Ceres decreases towards longer wavelengths. Such
distinct dielectric properties might be related to the hydrated phyllosilicate
composition of Ceres and possibly abundant $mu$m-sized grains on its surface.
The thermal inertia of Ceres is constrained by our modeling as likely being
between 40 and 160 tiu, much higher than previous measurements at infrared
wavelengths. Modeling also suggests that Ceres’s lightcurve is likely dominated
by spatial variations in its physical or compositional properties that cause
changes in Ceres’s observed thermal properties and dielectric absorption as it
rotates.

We observed Ceres at three epochs in 2015 November and 2017 September and
October with ALMA 12-meter array and in 2017 October with the ALMA Compact
Array (ACA), all at ~265 GHz continuum (wavelengths of ~1.1 mm) to map the
temperatures of Ceres over a full rotation at each epoch. We also used 2017
October ACA observations to search for HCN. The disk-averaged brightness
temperature of Ceres is measured to be between 170 K and 180 K during our 2017
observations. The rotational lightcurve of Ceres shows a double peaked shape
with an amplitude of about 4%. Our HCN search returns a negative result with an
upper limit production rate of ~2$times$10$^{24}$ molecules s$^{-1}$, assuming
globally uniform production and a Haser model. A thermophysical model suggests
that Ceres’s top layer has higher dielectric absorption than lunar-like
materials at a wavelength of 1 mm. However, previous observations showed that
the dielectric absorption of Ceres decreases towards longer wavelengths. Such
distinct dielectric properties might be related to the hydrated phyllosilicate
composition of Ceres and possibly abundant $mu$m-sized grains on its surface.
The thermal inertia of Ceres is constrained by our modeling as likely being
between 40 and 160 tiu, much higher than previous measurements at infrared
wavelengths. Modeling also suggests that Ceres’s lightcurve is likely dominated
by spatial variations in its physical or compositional properties that cause
changes in Ceres’s observed thermal properties and dielectric absorption as it
rotates.

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