216 research outputs found
Internal Motion of 6.7-GHz Methanol Masers in H II Region S269
We present the first internal motion measurement of the 6.7-GHz methanol
maser within S269, a small HII region in the outer Galaxy, which was carried
out in 2006 and 2011 using the Japanese VLBI Network (JVN). Several maser
groups and weak isolated spots were detected in an area spanning by ~200 mas
(1000 AU). Three remarkable maser groups are aligned at a position angle of 80
degree. Two of three maser groups were also detected by a previous observation
in 1998, which allowed us to study a long-term position variation of maser
spots from 1998 to 2011. The angular separation between the two groups
increased ~10 mas, which corresponds to an expansion velocity of ~10 km s^{-1}.
Some velocity gradient (~10^{-2} km s^{-1} mas^{-1}) in the overall
distribution was found. The internal motion between the maser groups support
the hypothesis that the methanol masers in S269 could trace a bipolar outflow.Comment: 8 pages, 6 figures, accepted for publication in PAS
Direct diagnostics of forming massive stars: stellar pulsation and periodic variability of maser sources
The 6.7 GHz methanol maser emission, a tracer of forming massive stars,
sometimes shows enigmatic periodic flux variations over several 10-100 days. In
this Letter, we propose that this periodic variations could be explained by the
pulsation of massive protostars growing under rapid mass accretion with rates
of Mdot > 10^-3 Msun/yr. Our stellar evolution calculations predict that the
massive protostars have very large radius exceeding 100 Rsun at maximum, and we
here study the pulsational stability of such the bloated protostars by way of
the linear stability analysis. We show that the protostar becomes pulsationally
unstable with various periods of several 10-100 days, depending on different
accretion rates. With the fact that the stellar luminosity when the star is
pulsationally unstable also depends on the accretion rate, we derive the
period-luminosity relation log (L/Lsun) = 4.62 + 0.98log(P/100 day), which is
testable with future observations. Our models further show that the radius and
mass of the pulsating massive protostar should also depend on the period. It
would be possible to infer such protostellar properties and the accretion rate
with the observed period. Measuring the maser periods enables a direct
diagnosis of the structure of accreting massive protostars, which are deeply
embedded in dense gas and inaccessible with other observations.Comment: 5 pages, 3 figures, 1 table, accepted for publication in ApJ
Physical activity, television viewing time and 12 year changes in waist circumference
PURPOSE: Both moderate-to-vigorous physical activity (MVPA) and sedentary behavior can be associated with adult adiposity. Much of the relevant evidence is from cross-sectional studies or from prospective studies with relevant exposure measures at a single time point prior to weight gain or incident obesity. This study examined whether changes in MVPA and television (TV) viewing time are associated with subsequent changes in waist circumference, using data from three separate observation points in a large population-based prospective study of Australian adults. METHODS: Data were obtained from the Australian Diabetes, Obesity and Lifestyle study collected in 1999-2000 (baseline), 2004-05 (Wave 2), and 2011-12 (Wave 3). The study sample consisted of adults aged 25 to 74 years at baseline who also attended site measurement at three time points (n=3261). Multilevel linear regression analysis examined associations of initial five-year changes in MVPA and TV viewing time (from baseline to Wave 2) with 12-year change in waist circumference (from baseline to Wave 3), adjusting for well-known confounders. RESULTS: As categorical predictors, increases in MVPA significantly attenuated increases in waist circumference (p for trend< 0.001). TV viewing time change was not significantly associated with changes in waist circumference (p for trend =0.06). Combined categories of MVPA and TV viewing time changes were predictive of waist circumference increases; compared to those who increased MVPA and reduced TV viewing time, those who reduced MVPA and increased TV viewing time had a 2cm greater increase in waist circumference (p=0.001). CONCLUSION: Decreasing MVPA emerged as a significant predictor of increases in waist circumference. Increasing TV viewing time was also influential, but its impact was much weaker than MVPA
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