A submillimetre survey for protostellar accretion discs using the JCMT–CSO interferometer

We present a submillimetre continuum survey for accretion discs around seven embedded protostars in the Perseus and Serpens molecular clouds. Observations were made at frequencies between 339 and 357 GHz using the James Clerk Maxwell Telescope–Caltech Submillimeter Observatory single-baseline interferometer on Mauna Kea, Hawaii. All the objects in our survey show compact dust emission on scales ≲1 arcsec, assumed to arise in a circumstellar accretion disc. We compare the properties of this compact component with evolutionary indicators, such as the ratio of compact to extended emission, and bolometric temperature. We find that discs of mass ∼0.01 M_⊙ have formed by the Class 0 stage, and that similar mass discs are observed in Class I and Class II sources. A trend is observed whereby the ratio of compact to extended emission in our sources increases from Class 0 to Class II sources. For three of the objects in the survey, NGC 1333 IRAS2:CR1 and SVS13 in Perseus, and FIRS1 in Serpens, the signal-to-noise ratio is sufficient to allow us to model the brightness distributions with elliptical Gaussian and power-law disc models. The Gaussian fits give semimajor half-power radii of approximately 90 to 140 au, at the assumed distance of 350 pc to the Perseus and Serpens clouds.     

Molecules, ices and astronomy

David A Williams, Wendy A Brown, Stephen D Price, Jonathan M C Rawlings and Serena Viti of University College London review how far experimental work on molecular synthesis can be applied to the interstellar medium – and how it is becoming a tool for the understanding of astrophysical processes.     

New generation CTD system [conductivity-temperature-depth sensor]

A microprocessor-based CTD (conductivity-temperature-depth) sensor unit is described in which the microprocessor is used to vary the analog-to-digital conversion process to achieve 16-bit resolution at a measurement rate of 500 measurements per second, or 18-bit resolution at 30 measurements per second. Software control of resolution, sampling rate, and sampling sequence permit the sensor unit to be software-configured for a wide variety of missions. Pressure and conductivity sensors are expected to substantially improve the system accuracy. The conductivity cell design has six electrodes which are all relatively remote from the external measurement volume (approximated by a 2-cm diameter sphere). This so called recessed-electrode concept is expected to eliminate calibration drift due to contamination of the electrodes and the insulating surfaces of the cell, thus ensuring excellent long-term stability     

Distribution of bottom stress and tidal energy dissipation in a well-mixed estuary

Estimates of area-averaged tidal bottom stress are made for four channel segments of the Great Bay Estuary, N.H. Current and sealevel measurements are used to estimate acceleration and pressure gradient terms in the equation of motion, while the equation of motion itself is used to infer the remaining stress term. Dynamic terms, bottom stress values, friction coefficients and energy dissipation rates are estimated for each site. The analysis shows that while throughout the estuary the principal force balance is between the frictional stress and the pressure gradient forcing, RMS values of total bottom stress range from 2·67 to 10·38 Nm^?2 and friction coefficients vary from 0·015 to 0·054. Both stress and energy dissipation are largest in the seaward portion of the estuary with an order of magnitude decrease in dissipation at the most inland site.These distributions of stress and energy dissipation are consistent with cotidal charts of the principal semi-diurnal tidal constituent (M₂) which indicate that the estuary is composed of a highly dissipative more progressive tidal wave regime seaward and a less dissipative standing wave regime landward.