Space instrumentation

Abstract:

[1] Observations of Venus' mesosphere by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M instrument of Venus Express have been used to investigate the spatial distribution of CO above Venus' nightside cloud tops by fitting the CO absorption in the (1-0) CO band around 4.7 μm. We find little spatial variation in the abundance of CO at midlatitudes, with a retrieved abundance of approximately 40 ± 10 ppm just above the cloud tops between 65 and 70 km altitude. Unfortunately, we find it very difficult to constrain the abundance of CO in the cold polar collar, centered at about 70°S, as the retrieved temperature structure in the CO line-forming region masks the absorption lines. However, there is a possibility that CO increases toward the poles, as we detect a significant signature of high levels of CO over Venus' south polar dipole feature in all the observations analyzed so far. To constrain the abundance of CO more closely will require the analysis of higher-resolution VIRTIS-H observations. In addition, limb observations would greatly help to resolve any possible temperature/cloud ambiguities and allow us to assess vertical variations in the abundance of CO. Copyright 2008 by the American Geophysical Union.

Abstract:

The aim of the Degenerate Objects around Degenerate Objects (DODO) survey is to search for very low-mass brown dwarfs and extrasolar planets in wide orbits around white dwarfs via direct imaging. The direct detection of such companions would allow the spectroscopic investigation of objects with temperatures much lower (<500 K) than the coolest brown dwarfs currently observed. These ultra-low-mass substellar objects would have spectral types >T8.5, and so could belong to the proposed Y dwarf spectral sequence. The detection of a planet around a white dwarf would prove that such objects can survive the final stages of stellar evolution and place constraints on the frequency of planetary systems around their progenitors (with masses between 1.5 and 8 M⊙, i.e. early B to mid-F). This paper presents the results of a multi epoch J band common proper motion survey of 23 nearby equatorial and Northern hemisphere white dwarfs. We rule out the presence of any common proper motion companions, with limiting masses determined from the completeness limit of each observation, to 18 white dwarfs. For the remaining five targets, the motion of the white dwarf is not sufficiently separated from the non-moving background objects in each field. These targets require additional observations to conclusively rule out the presence of any common proper motion companions. From our completeness limits, we tentatively suggest that ≲5 per cent of white dwarfs have substellar companions with T eff ≳ 500 K between projected physical separations of 60-200 au. © 2009 The Authors. © 2009 RAS.

Abstract:

[1] We present nightside observations of tropospheric carbon monoxide in the southern hemisphere near the 35 km height level, the first from Venus Express/Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)-M-IR. VIRTIS-M data from 2.18 to 2.50 μm, with a spectral resolution of 10 nm, were used in the analysis. Spectra were binned, with widths ranging from 5 to 30 spatial pixels, to increase the signal-to-noise ratio, while at the same time reducing the total number of retrievals required for complete spatial coverage. We calculate the mean abundance for carbon monoxide at the equator to be 23 ± 2 ppm. The CO concentration increases toward the poles, peaking at a latitude of approximately 60°S, with a mean value of 32 ± 2 ppm. This 40% equator-to-pole increase is consistent with the values found by Collard et al. (1993) from Galileo/NIMS observations. Observations suggest an overturning in this CO gradient past 60°S, declining to abundances seen in the midlatitudes. Zonal variability in this peak value has also been measured, varying on the order of 10% (∼3 ppm) at different longitudes on a latitude circle. The zonal variability of the CO abundance has possible implications for the lifetime of CO and its dynamics in the troposphere. This work has definitively established a distribution of tropospheric CO, which is consistent with a Hadley cell circulation, and placed limits on the latitudinal extent of the cell. Copyright 2008 by the American Geophysical Union.