3 research outputs found
Discovery of Negative Superhumps during a Superoutburst of January 2011 in ER Ursae Majoris
We report on a discovery of "negative" superhumps during the 2011 January
superoutburst of ER UMa. During the superoutburst which started on 2011 January
16, we detected negative superhumps having a period of 0.062242(9) d, shorter
than the orbital period by 2.2%. No evidence of positive superhumps was
detected during this observation. This finding indicates that the disk
exhibited retrograde precession during this superoutburst, contrary to all
other known cases of superoutbursts. The duration of this superoutburst was
shorter than those of ordinary superoutbursts and the intervals of normal
outbursts were longer than ordinary ones. We suggest a possibility that such
unusual outburst properties are likely a result of the disk tilt, which is
supposed to be a cause of negative superhumps: the tilted disk could prevent
the disk from being filled with materials in the outmost region which is
supposed to be responsible for long-duration superoutbursts in ER UMa-type
dwarf novae. The discovery signifies the importance of the classical prograde
precession in sustaining long-duration superoutbursts. Furthermore, the
presence of pronounced negative superhumps in this system with a high
mass-transfer rate favors the hypothesis that hydrodynamical lift is the cause
of the disk tilt.Comment: 8 pages, 3 figures, Accepted for publication in PASJ Lette
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Permeability, electrical impedance, and acoustic velocities on reservoir rocks from the Geysers geothermal field
Previous measurements of acoustic velocities on NEGU- 17 cores indicate that saturation effects are significant enough to cause Vp/Vs anomalies observed in the field. In this study we report on the results of new measurements on core recently recovered from SB-15-D along with some additional measurements on the NEGU-17 cores. The measurements indicate correlations between mechanical, transport, and water storage properties of the matrix which may prove useful for reservoir assessment and management. The SB-15-D material is found to be similar to the NEGU-17 material in terms of acoustic velocities, being characterized by a notably weak pressure dependence on the velocities and a modest Vp/Vs signature of saturation. The effect of saturation on Vp/Vs appears to result in part from a chemo-mechanical weakening of the shear modulus due to the presence of water. Electrical properties of SB-15-D material are qualitatively similar to those of the NEGU-17 cores, although resistivities of SB-15-D cores are notably lower and dielectric permittivities higher than in their NEGU- 17 counterparts. While some limited correlations of measured properties with depth are noted, no clear change in character is observed within SB-15-D cores which can be associated with the proposed cap-rock/reservoir boundary