Stellar Observations Network Group (SONG): SONG in New Mexico

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song_science [2023/08/04 20:23] – [SONG project status] jasonjsong_science [2023/08/05 14:43] (current) – [How asteroseismology works] jasonj
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 ===== How asteroseismology works ===== ===== How asteroseismology works =====
-{{ :science:four_modes.gif?direct|}}+
  
 We focus on stars that pulsate like our Sun in what follows, both for simplicity and because the interpretation of the data is much more advanced than for other types of pulsators. Such solar-like oscillators need not be solar-type, main-sequence stars at all; for example, almost all red giants, such as the bright Aldebaran {[farr2018]}, display solar-like oscillations. They are the result of acoustic pressure waves that are excited to small, yet observable amplitudes by near-surface turbulent convection {[goldreich1977]}.  These stars, therefore,  must have an outer convection zone, with effective (surface) temperature below about 7000K, corresponding to an upper mass of about 1.5 solar masses on the main sequence and spectral types later than mid-F. For evolved  subgiants and giants, G, K, and M stars are most typical (and masses can exceed 1.5 solar masses). We focus on stars that pulsate like our Sun in what follows, both for simplicity and because the interpretation of the data is much more advanced than for other types of pulsators. Such solar-like oscillators need not be solar-type, main-sequence stars at all; for example, almost all red giants, such as the bright Aldebaran {[farr2018]}, display solar-like oscillations. They are the result of acoustic pressure waves that are excited to small, yet observable amplitudes by near-surface turbulent convection {[goldreich1977]}.  These stars, therefore,  must have an outer convection zone, with effective (surface) temperature below about 7000K, corresponding to an upper mass of about 1.5 solar masses on the main sequence and spectral types later than mid-F. For evolved  subgiants and giants, G, K, and M stars are most typical (and masses can exceed 1.5 solar masses).
  
-The oscillations are global modes in a star, which distort the stellar surface with a spatial pattern that can usually be described by spherical harmonics, resulting in luminosity and radial-velocity variations. The figure shows 4 examples, where the red and blue denote the distortion of the particular part of the surface, and white are nodes (no distortion). Cancellation effects due to the point-source nature of distant stars only allow for observations of the lowest spherical harmonic degrees (L=0 - 3). In the figure, the top 2 animations are for L=1 and L=3, which would be observable. The other two (L=6 and L=10, would not be). Power spectra of a time series of a solar-like oscillator show a comb-like structure of peaks within a broad acoustic mode envelope that has a maximum amplitude at some temporal frequency. This can range from about 20 μHz (half a day period) for evolved giants to a few thousand μHz (periods of minutes) for dwarfs.  The comb pattern has peaks that are evenly spaced in frequency, whereby frequency differences between modes of consecutive radial order n and the same spherical degree L are known as the large frequency spacing.  +{{ :science:four_modes.gif?direct|}} 
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 +The oscillations are global modes in a star, which distort the stellar surface with a spatial pattern that can usually be described by spherical harmonics, resulting in luminosity and radial-velocity variations. The figure shows 4 examples, where the red and blue denote the (highly exaggerated) distortion of the particular part of the surface, and white are nodes (no distortion). Cancellation effects due to the point-source nature of distant stars only allow for observations of the lowest spherical harmonic degrees (L=0 - 3). In the figure, the top 2 animations are for L=1 and L=3, which would be observable. The other two (L=6 and L=10, would not be). Power spectra of a time series of a solar-like oscillator show a comb-like structure of peaks within a broad acoustic mode envelope that has a maximum amplitude at some temporal frequency. This can range from about 20 μHz (half a day period) for evolved giants to a few thousand μHz (periods of minutes) for dwarfs.  The comb pattern has peaks that are evenly spaced in frequency, whereby frequency differences between modes of consecutive radial order n and the same spherical degree L are known as the large frequency spacing.  
  
 These observed modal properties are often interpreted in terms of the asymptotic theory of stellar oscillations {[tassoul1980]}. In this case, the large frequency spacing is related to the sound crossing time of an acoustic wave across the star, and therefore scales with the mean density. An empirically-motivated relationship connects the frequenc of maximum power with the surface gravity and effective temperature {[brown1991]}. When these two relations are combined, scaling relations for a star's mass and radius can be derived {[kjeldsen1995]} These observed modal properties are often interpreted in terms of the asymptotic theory of stellar oscillations {[tassoul1980]}. In this case, the large frequency spacing is related to the sound crossing time of an acoustic wave across the star, and therefore scales with the mean density. An empirically-motivated relationship connects the frequenc of maximum power with the surface gravity and effective temperature {[brown1991]}. When these two relations are combined, scaling relations for a star's mass and radius can be derived {[kjeldsen1995]}
song_science.1691180586.txt.gz · Last modified: 2023/08/04 20:23 by jasonj