Distant field blue horizontal branch stars – III. Identification of a probable outer halo stream at Galactocentric distance r= 70 kpc

We present FOcal Reducer/low dispersion Spectrograph-1 spectra (from the European Southern Observatory's Very Large Telescope) of a sample of 34 faint  20.0 < g * < 21.1  A-type stars selected from the Sloan Digital Sky Survey Early Data Release, with the goal of measuring the velocity dispersion of blue horizontal branch (BHB) stars in the remote Galactic halo,   R ∼ 80 kpc  . We show that colour selection with  1.08 < u *− g * < 1.40  and  −0.2 < g *− r * < −0.04  minimizes contamination of the sample by less luminous blue stragglers. In classifying the stars we confine our attention to the 20 stars with spectra of signal-to-noise ratio >15 Å⁻¹. Classification produces a sample of eight BHB stars at distances  65–102 kpc  from the Sun (mean 80 kpc), which represents the most distant sample of Galactic stars with measured radial velocities. The dispersion of the measured radial component of the velocity with respect to the centre of the Galaxy is  58 ± 15 km s⁻¹  . This value is anomalously low in comparison with measured values for stars at smaller distances, as well as for satellites at similar distances. Seeking an explanation for the low measured velocity dispersion, further analysis reveals that six of the eight remote BHB stars are plausibly associated with a single orbit. Three previously known outer halo carbon stars also appear to belong to this stream. The velocity dispersion of all nine stars relative to the orbit is only  15 ± 4 km s⁻¹  . Further observations along the orbit are required to trace the full extent of this structure on the sky.     

Tracing the star stream through M31 using planetary nebula kinematics

We present a possible orbit for the Southern Stream of stars in M31, which connects it to the Northern Spur. Support for this model comes from the dynamics of planetary nebulae (PNe) in the disc of M31: analysis of a new sample of 2611 PNe obtained using the Planetary Nebula Spectrograph reveals ∼20 objects with kinematics inconsistent with the normal components of the galaxy, but which lie at the right positions and velocities to connect the two photometric features via this orbit. The satellite galaxy M32 is coincident with the stream both in position and velocity, adding weight to the hypothesis that the stream comprises its tidal debris.     

The treatment of observational radio-meteor wind data

Contributions to the observed scatter in wind velocity measurement from radiometeors in the 80–110 km height region are investigated in the light of results obtained with a phase coherent pulsed radar system. The major sources of variance considered are the analysis technique, aerial effects and atmospheric variations. It is of importance for published wind data to indicate the significance of derived mean wind velocity values.     

Observations of the infrared triplet of singly ionized calcium

Observations are presented of the Caii infrared triplet (8498 Å, 8542 Å, and 8662 Å) at three positions on the solar disk to make possible direct analyses of the lines and comparisons with theoretical computations. The source functions for the two strongest lines (8542 Å and 8662 Å) are equal at those heights corresponding to the wings of the lines (¦¦ > 0.4 Å) but not to those of the cores. We suggest that the apparent source function inequality in the cores is due to limb darkening caused by inhomogeneities in the chromosphere.Of the National Bureau of Standards and University of Colorado.     

Atmospheric circulation and storminess derived from Royal Navy logbooks: 1685 to 1750

This paper is concerned with the use of Royal Navy ships’ logbooks for the period 1685 to 1750, thereby embracing the oldest members of this documentary series that extends to the present day. The geographic range of the undertaking is confined to the English Channel and its western approaches where the abundance of logbooks for this period allows for the abstraction of a daily series of wind force and direction data. These are verified and processed for inclusion in a database from which indices are derived for air circulation patterns based on the frequency of winds from north, east, south and west quarters, and of gale frequency, based on the contemporary terminology of wind force. The methods by which the data are abstracted and processed are described, as is the nature of the raw data and source material. The results provide a uniquely detailed insight into the changing patterns of air circulation over this critical period that marked the transition from the depths of the Little Ice Age in the late seventeenth century. Attention is also drawn to the changing nature of gale frequency, which revealed a notable decrease over the study period. Associations between changing circulation patterns and temperature regimes are also explored.     

Merging time-scales of stellar subclumps in young star-forming regions

Recent observations and hydrodynamical simulations of star formation inside a giant molecular cloud have revealed that, within a star-forming region, stars do not form evenly distributed throughout this region, but rather in small subclumps. It is generally believed that these subclumps merge and form a young star cluster. The time-scale of this merging process is crucial for the evolution and the possible survival of the final star cluster. The key issue is whether this merging process happens faster than the time needed to remove the residual gas of the cloud. A merging time-scale shorter than the gas-removal time would enhance the survival chances of the resulting star cluster. In this paper, we show by means of numerical simulations that the time-scale of the merging is indeed very fast. Depending on the details of the initial subclump distribution, the merging may occur before the gas is expelled from the newly formed cluster via either supernovae or the winds from massive stars. Our simulations further show that the resulting merger objects have a higher effective star formation efficiency than the overall star-forming region and confirm the results that mass-segregated subclumps form mass-segregated merger objects.