| 1. |
- Metcalfe, Travis S., et al.
(författare)
-
Constraints on Magnetic Braking from the G8 Dwarf Stars 61 UMa and tau Cet
- 2023
-
Ingår i: Astrophysical Journal Letters. - : Institute of Physics (IOP). - 2041-8205 .- 2041-8213. ; 948:1
-
Tidskriftsartikel (refereegranskat)abstract
- During the first half of their main-sequence lifetimes, stars rapidly lose angular momentum to their magnetized winds, a process known as magnetic braking. Recent observations suggest a substantial decrease in the magnetic braking efficiency when stars reach a critical value of the Rossby number, the stellar rotation period normalized by the convective overturn timescale. Cooler stars have deeper convection zones with longer overturn times, reaching this critical Rossby number at slower rotation rates. The nature and timing of the transition to weakened magnetic braking have previously been constrained by several solar analogs and two slightly hotter stars. In this Letter, we derive the first direct constraints from stars cooler than the Sun. We present new spectropolarimetry of the old G8 dwarf tau Cet from the Large Binocular Telescope, and we reanalyze a published Zeeman Doppler image of the younger G8 star 61 UMa, yielding the large-scale magnetic field strengths and morphologies. We estimate mass-loss rates using archival X-ray observations and inferences from Ly alpha measurements, and we adopt other stellar properties from asteroseismology and spectral energy distribution fitting. The resulting calculations of the wind braking torque demonstrate that the rate of angular momentum loss drops by a factor of 300 between the ages of these two stars (1.4-9 Gyr), well above theoretical expectations. We summarize the available data to help constrain the value of the critical Rossby number, and we identify a new signature of the long-period detection edge in recent measurements from the Kepler mission.
|
|
| 2. |
- Mueller-Blenkle, Christina, et al.
(författare)
-
Effects of pile-driving noise on the behaviour of marine fish
- 2010
-
Rapport (övrigt vetenskapligt/konstnärligt)abstract
- Studies on the effects of offshore wind farm construction on marine life have so far focussed on behavioural reactions in porpoises and seals. The effects on fish have only very recently come into the focus of scientists, regulators and stakeholders. Pile-driving noise during construction is of particular concern as the very high sound pressure levels could potentially prevent fish from reaching breeding or spawning sites, finding food, and acoustically locating mates. This could result in longterm effects on reproduction and population parameters. Further, avoidance reactions might result in displacement away from potential fishing grounds and lead to reduced catches. However, reaction thresholds and therefore the impacts of pile-driving on the behaviour of fish are completely unknown. We played back pile-driving noise to cod and sole held in two large (40 m) net pens located in a quiet Bay in West Scotland. Movements of the fish were analysed using a novel acoustic tracking system. Received sound pressure level and particle motion were measured during the experiments. There was a significant movement response to the pile-driving stimulus in both species at relatively low received sound pressure levels (sole: 144 – 156 dB re 1μPa Peak; cod: 140 – 161 dB re 1 μPa Peak, particle motion between 6.51x10-3 and 8.62x10-4 m/s2 peak). Sole showed a significant increase in swimming speed during the playback period compared to before and after playback. Cod exhibited a similar reaction, yet results were not significant. Cod showed a significant freezing response at onset and cessation of playback. There were indications of directional movements away from the sound source in both species. The results further showed a high variability in behavioural reactions across individuals and a decrease of response with multiple exposures. This study is the first to document behavioural response of marine fish due to playbacks of pile-driving sounds. The results indicate that a range of received sound pressure and particle motion levels will trigger behavioural responses in sole and cod. The results further imply a relatively large zone of behavioural response to pile-driving sounds in marine fish. Yet, the exact nature and extent of the behavioural response needs to be investigated further. Some of our results point toward habituation to the sound. The results of the study have important implications for regulatory advice and the implementation of mitigation measures in the construction of offshore wind farms in the UK and elsewhere. First, the concerns raised about the potential effects of pile-driving noise on fish were well founded. This suggests to both regulators and developers that the costs imposed by some mitigation measures that have so far been applied following the precautionary principle go some of the way to addressing a real problem. We also suggest that our behavioural thresholds are considered in assessments of impacts of offshore wind farms in the UK and elsewhere. Mitigation measures should be further discussed developed and, if meaningful, applied especially if these could lead to a reduction of acoustic energy that is emitted into the water column. Further studies should investigate the response at critical times (e.g. mating and spawning) and the effects of pile-driving on communication behaviour. It will also be necessary to further investigate habituation to the sound to effectively manage effects of pile-driving sound on marine fish.
|
|
| 3. |
|
|
