36,518 research outputs found
The rotation rates of massive stars: How slow are the slow ones?
Context: Rotation plays a key role in the life cycles of stars with masses
above 8 Msun. Hence, accurate knowledge of the rotation rates of such massive
stars is critical for understanding their properties and for constraining
models of their evolution. Aims: This paper investigates the reliability of
current methods used to derive projected rotation speeds v sin i from
line-broadening signatures in the photospheric spectra of massive stars,
focusing on stars that are not rapidly rotating. Methods: We use slowly
rotating magnetic O-stars with well-determined rotation periods to test the
Fourier transform (FT) and goodness-of-fit (GOF) methods typically used to
infer projected rotation rates of massive stars. Results: For our two magnetic
test stars with measured rotation periods longer than one year, i.e., with v
sin i < 1 km/s, we derive v sin i ~ 40-50 km/s from both the FT and GOF
methods. These severe overestimates are most likely caused by an insufficient
treatment of the competing broadening mechanisms referred to as microturbulence
and macroturbulence. Conclusions: These findings warn us not to rely
uncritically on results from current standard techniques to derive projected
rotation speeds of massive stars in the presence of significant additional line
broadening, at least when v sin i <~ 50 km/s. This may, for example, be crucial
for i) determining the statistical distribution of observed rotation rates of
massive stars, ii) interpreting the evolutionary status and spin-down histories
of rotationally braked B-supergiants, and iii) explaining the deficiency of
observed O-stars with spectroscopically inferred v sin i ~ 0 km/s. Further
investigations of potential shortcomings of the above techniques are presently
under way.Comment: 4 pages, 4 figures, accepted for publication in A&A Letter
Water and energy footprint of irrigated agriculture in the Mediterranean region
Irrigated agriculture constitutes the largest consumer of freshwater in the Mediterranean region and provides a major source of income and employment for rural livelihoods. However, increasing droughts and water scarcity have highlighted concerns regarding the environmental sustainability of agriculture in the region. An integrated assessment combining a gridded water balance model with a geodatabase and GIS has been developed and used to assess the water demand and energy footprint of irrigated production in the region. Modelled outputs were linked with crop yield and water resources data to estimate water (m3 kg−1) and energy (CO2 kg−1) productivity and identify vulnerable areas or 'hotspots'. For a selected key crops in the region, irrigation accounts for 61 km3 yr−1 of water abstraction and 1.78 Gt CO2 emissions yr−1, with most emissions from sunflower (73 kg CO2/t) and cotton (60 kg CO2/t) production. Wheat is a major strategic crop in the region and was estimated to have a water productivity of 1000 t Mm−3 and emissions of 31 kg CO2/t. Irrigation modernization would save around 8 km3 of water but would correspondingly increase CO2 emissions by around +135%. Shifting from rain-fed to irrigated production would increase irrigation demand to 166 km3 yr−1 (+137%) whilst CO2 emissions would rise by +270%. The study has major policy implications for understanding the water–energy–food nexus in the region and the trade-offs between strategies to save water, reduce CO2 emissions and/or intensify food production
On the use of the Fourier Transform to determine the projected rotational velocity of line-profile variable B stars
The Fourier Transform method is a popular tool to derive the rotational
velocities of stars from their spectral line profiles. However, its domain of
validity does not include line-profile variables with time-dependent profiles.
We investigate the performance of the method for such cases, by interpreting
the line-profile variations of spotted B stars, and of pulsating B tars, as if
their spectral lines were caused by uniform surface rotation along with
macroturbulence. We perform time-series analysis and harmonic least-squares
fitting of various line diagnostics and of the outcome of several
implementations of the Fourier Transform method. We find that the projected
rotational velocities derived from the Fourier Transform vary appreciably
during the pulsation cycle whenever the pulsational and rotational velocity
fields are of similar magnitude. The macroturbulent velocities derived while
ignoring the pulsations can vary with tens of km/s during the pulsation cycle.
The temporal behaviour of the deduced rotational and macroturbulent velocities
are in antiphase with each other. The rotational velocity is in phase with the
second moment of the line profiles. The application of the Fourier method to
stars with considerable pulsational line broadening may lead to an appreciable
spread in the values of the rotation velocity, and, by implication, of the
deduced value of the macroturbulence. These two quantities should therefore not
be derived from single snapshot spectra if the aim is to use them as a solid
diagnostic for the evaluation of stellar evolution models of slow to moderate
rotators.Comment: 13 pages, 9 figures, accepted for publication in Astronomy &
Astrophysic
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