34 research outputs found
Arboreal forage lichen response to partial cutting of high elevation mountain caribou range in the Quesnel Higland of east-central British Columbia
Group selection silvicultural systems have been recommended for managing mountain caribou (Rangifer tarandus caribou) habitat in high elevation Engelmann spruce – subalpine fir forests in east-central British Columbia. We measured the response of arboreal lichen (a key winter forage) to harvesting of 30% of the forested area using three partial cutting treatments, which created small (0.03 ha), medium (0.13 ha), and large (1.0 ha) openings, and a no-harvest treatment. Treatments were replicated on four sites, and monitored over a ten year post-harvest period. The short-term loss of lichen associated with removal of approximately one third of the trees was partially offset by a significant (P=0.01) increase in lichen abundance on trees in the caribou feeding zone (up to 4.5 m) in the three partial cutting treatments relative to trees in the uncut forest. Differences among treatments in the change in lichen composition, as measured by the percentage of Alectoria sarmentosa and Bryoria spp., were marginally significant (P=0.10). The partial cutting treatments showing a greater likelihood of shifting towards more Bryoria spp. than no-harvest treatment (P=0.04). In the year of harvest (1993), larger trees were found to hold more lichen than smaller trees (P=0.04), and live trees supported more lichen than dead trees (P=0.01), but lichen loading was similar among tree species (P=0.51). Tree fall rates were similar among treatments, based on the ten year average (0.6–0.8% of sample trees per year). The results indicate that caribou foraging habitat is maintained in the residual forest when group selection systems that remove only 30% of the trees are applied. Information on the distribution of lichen is useful for developing stand level prescriptions. Providing lichen bearing habitat meets just one of the needs of caribou. A comprehensive approach that considers all factors and their interactions is essential to maintain and recover the threatened mountain caribou
The statistical significance of the superhump signal in U Gem
Although its well determined mass ratio of q=\Msec/\Mwd=0.357\pm0.007
should avoid superoutbursts according to the thermal tidal instability model,
the prototypical dwarf nova U Gem experienced in 1985 an extraordinary long
outburst resembling very much superoutbursts observed in SU UMa systems.
Recently, the situation for the model became even worse as superhump detections
have been reported for the 1985 outburst of U Gem. The superhump signal is
noisy and the evidence provided by simple periodograms seems to be weak.
Therefore and because of the importance for our understanding of superoutbursts
and superhumps, we determine the statistical significance of the recently
published detection of superhumps in the AAVSO light curve of the famous long
1985 outburst of U Gem. Using Lomb-Scargle periodograms, analysis of variance
(AoV), and Monte-Carlo methods we analyse the 160 visual magnitudes obtained by
the AAVSO during the outburst and relate our analyse to previous superhump
detections. The 160 data points of the outburst alone do not contain a
statistically significant period. However, using additionally the
characteristics of superhumps detected previously in other SU UMa systems and
searching only for signals that are consistent with these, we derive a
significance for the superhump signal. The alleged appearance of an
additional superhump at the end of the outbursts appears to be statistically
insignificant. Although of weak statistical significance, the superhump signal
of the long 1985 outburst of U Gem can be interpreted as further indication for
the SU UMa nature of this outburst. This further contradicts the tidal
instability model as the explanation for the superhump phenomenon.Comment: 7 pages, 7 figures, accepted for publication in A&
Dynamics of the solar chromosphere. V. High-frequency modulation in ultraviolet image sequences from TRACE
We search for signatures of high-frequency oscillations in the upper solar
photosphere and low chromosphere in the context of acoustic heating of outer
stellar atmospheres. We use ultraviolet image sequences of a quiet center-disk
area from the Transition Region and Coronal Explorer (TRACE) mission which were
taken with strict cadence regularity. The latter permits more reliable
high-frequency diagnosis than in earlier work. Spatial Fourier power maps,
spatially averaged coherence and phase-difference spectra, and spatio-temporal
k-f decompositions all contain high-frequency features that at first sight seem
of considerable intrinsic interest but actually are more likely to represent
artifacts of different nature. Spatially averaged phase difference measurement
provides the most sensitive diagnostic and indicates the presence of acoustic
modulation up to f=20 mHz (periods down to 50 seconds) in internetwork areas.Comment: 9 pages, 8 figure
Observation of multiple sausage oscillations in cool postflare loop
Using simultaneous high spatial (1.3 arc sec) and temporal (5 and 10 s)
resolution H-alpha observations from the 15 cm Solar Tower Telescope at ARIES,
we study the oscillations in the relative intensity to explore the possibility
of sausage oscillations in the chromospheric cool postflare loop. We use
standard wavelet tool, and find the oscillation period of ~ 587 s near the loop
apex, and ~ 349 s near the footpoint. We suggest that the oscillations
represent the fundamental and the first harmonics of fast sausage waves in the
cool postflare loop. Based on the period ratio P1/P2 ~ 1.68, we estimate the
density scale height in the loop as ~ 17 Mm. This value is much higher than the
equilibrium scale height corresponding to H-alpha temperature, which probably
indicates that the cool postflare loop is not in hydrostatic equilibrium.
Seismologically estimated Alfv\'en speed outside the loop is ~ 300-330 km/s.
The observation of multiple oscillations may play a crucial role in
understanding the dynamics of lower solar atmosphere, complementing such
oscillations already reported in the upper solar atmosphere (e.g., hot flaring
loops).Comment: 13 pages, 4 figures, accepted in MNRA
Extrasolar planets and brown dwarfs around A-F type stars. IV. A candidate brown dwarf around the A9V pulsating star HD180777
We present here the detection of a brown dwarf orbiting the A9V star
HD180777. The radial velocity measurements, obtained with the ELODIE echelle
spectrograph at the Haute-Provence Observatory, show a main variation with a
period of 28.4 days. Assuming a primary mass of 1.7 Mo, the best Keplerian fit
to the data leads to a minimum mass of 25 MJup for the companion (the true mass
could be significantly higher). We also show that, after substraction of the
Keplerian solution from the radial velocity measurements, the residual radial
velocities are related to phenomena intrinsic to the star, namely pulsations
with typical periods of Gamma Dor stars. These results show that in some cases,
it is possible to disentangle radial velocity variations due to a low mass
companion from variations intrinsic to the observed star.Comment: 7 pages, final version, accepted for publication (A&A
The Periodic Spectroscopic Variability of FU Orionis
FU Orionis systems are young stars undergoing outbursts of disc accretion and
where the optical spectrum contains lines associated with both the disc
photosphere and a wind component. Previous observations of the prototype FU
Orionis have suggested that the wind lines and the photospheric lines are
modulated with periods of 14.54 and 3.54 days respectively (Herbig et al.
2003). We have re-observed the system at higher spectral resolution, by
monitoring variations of optical line profiles over 21 nights in 2007 and have
found periods of 13.48 and 3.6 days in the wind and disc components consistent
with the above: this implies variability mechanisms that are stable over at
least a decade. In addition we have found: i) that the variations in the
photospheric absorption lines are confined to the blue wing of the line (around
-9km/s): we tentatively ascribe this to an orbiting hotspot in the disc which
is obscured by a disc warp during its receding phase. ii) The wind period is
manifested not only in blue-shifted Halpha absorption, but also in red-shifted
emission of Halpha and Hbeta, as well as in blue-shifted absorption of Na I D,
Li I and Fe II. iii) We find that the periodic modulation of blue-shifted
Halpha absorption at around -100km/s, is phase lagged with respect to
variations in the other lines by ~1.8days. This is consistent with a picture in
which variations at the wind base first affect chromospheric emission and then
low velocity blue-shifted absorption, followed - after a lag equal to the
propagation time of disturbances across the wind's acceleration region - by a
response in high velocity blue-shifted absorption. Such arguments constrain the
size of the acceleration region to ~10^12cm. We discuss possible mechanisms for
periodic variations within the innermost 0.1AU of the disc, including the
possibility that these variations indicate the presence of an embedded hot
Jupiter.Comment: 20 pages, 23 figures. Accepted for publication in MNRAS. See
http://www.ast.cam.ac.uk/~slp65/FUOripaperHRes.pdf for a pdf version of the
paper with high-resolution images; footnote added to the titl
Rotation of young stars in Cepheus OB3b
We present a photometric study of I-band variability in the young association Cepheus OB3b. The study is sensitive to periodic variability on time-scales of less than a day, to more than 20 d. After rejection of contaminating objects using V, I, R and narrow-band Hα photometry, we find 475 objects with measured rotation periods, which are very likely pre-main-sequence members of the Cep OB3b star-forming region.
We revise the distance and age to Cep OB3b, putting it on the self-consistent age and distance ladder of Mayne & Naylor. This yields a distance modulus of 8.8 ± 0.2 mag, corresponding to a distance of 580 ± 60 pc, and an age of 4–5 Myr.
The rotation period distribution confirms the general picture of rotational evolution in young stars, exhibiting both the correlation between accretion (determined in this case through narrow-band Hα photometry) and rotation expected from disc locking, and the dependence of rotation upon mass that is seen in other star-forming regions. However, this mass dependence is much weaker in our data than found in other studies. Comparison to the similarly aged NGC 2362 shows that the low-mass stars in Cep OB3b are rotating much more slowly. This points to a possible link between star-forming environment and rotation properties. Such a link would call into question models of stellar angular momentum evolution, which assume that the rotational period distributions of young clusters and associations can be assembled into an evolutionary sequence, thus ignoring environmental effects