Can you help me "translate" this abstract?

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Ann
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Can you help me "translate" this abstract?

Post by Ann » Thu Jun 30, 2011 9:32 pm

I found what I think is an interesting paper on http://arxiv.org/list/astro-ph/new, but as usual, the abstract is rather too "abstract" for me! I would be very happy if someone could help me "translate" it. I have marked the parts of it that I would be particularly happy to have explained to me:
Based on data for 102 OB3 stars with known proper motions and radial velocities, we have tested the distances derived by Megier et al. from interstellar Ca II spectral lines. The internal reconciliation of the distance scales using the first derivative of the angular velocity of Galactic rotation {\Omega}'0 and the external reconciliation with Humphreys's distance scale for OB associations refined by Mel'nik and Dambis show that the initial distances should be reduced by \approx 20%. Given this correction, the heliocentric distances of these stars lie within the range 0.6-2.6 kpc. A kinematic analysis of these stars at a fixed Galactocentric distance of the Sun, R0=8 kpc, has allowed the following parameters to be determined:(1) the solar peculiar velocity components (U_o,V_o,W_o)=(8.9,10.3,6.8)\pm(0.6,1.0,0.4) km/s;(2) the Galactic rotation parameters {\Omega}_o=-31.5\pm0.9 km/s/kpc, {\Omega}'_o=+4.49\pm0.12 km/s/kpc^2, {\Omega}"_o=-1.05\pm0.38 km/s/kpc^3, (the corresponding Oort constants are A=17.9\pm0.5 km/s/kpc, B=-13.6\pm1.0 km/s/kpc and the circular rotation velocity of the solar neighborhood is |V_o|=252\pm14 km/s); (3) the spiral density wave parameters, namely: the perturbation amplitudes for the radial and azimuthal velocity components, respectively, f_R = -12.5\pm1.1 km/s and f_{\theta}=2.0\pm1.6 km/s; the pitch angle for the two-armed spiral pattern i=-5.3\pm0.3 degrees, with the wavelength of the spiral density wave at the solar distance being {\lambda}=2.3\pm0.2 kpc; the Sun's phase in the spiral wave {\chi}_o=-91\pm4 degrees.
Anyone?

The full paper is here: http://arxiv.org/pdf/1106.5849

Ann
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neufer
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Re: Can you help me "translate" this abstract?

Post by neufer » Thu Jun 30, 2011 10:34 pm

When I was in graduate school in Astronomy at the Univ. of Maryland we were assigned the task of calculating the solar peculiar velocity based upon measurements of the average general relative motion of nearby stars. This study goes far beyond our simple student analysis by looking for both the overall rotation and divergence of nearby stars. This provides information about both galactic rotation and galactic density waves (which maintain the spiral structure).
Art Neuendorffer

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Re: Can you help me "translate" this abstract?

Post by Ann » Fri Jul 01, 2011 5:57 am

Thanks, Art. I think I understand, at least in general terms, the concept of "peculiar velocity" (and I don't necessarily think that the solar peculiar velocity is exactly "peculiar"). I don't understand, however, what the (U_o,V_o,W_o)=(8.9,10.3,6.8)\pm(0.6,1.0,0.4) km/s means. :?:

I understand what it means, more or less, that
the circular rotation velocity of the solar neighborhood is |V_o|=252\pm14 km/s)
That ought to mean, more or less, that the general rotational speed of things in the part of the galaxy where the Sun is located is, by and large, 252 km/second. :!:

I don't, however, understand what it means that
the Galactic rotation parameters {\Omega}_o=-31.5\pm0.9 km/s/kpc, {\Omega}'_o=+4.49\pm0.12 km/s/kpc^2, {\Omega}"_o=-1.05\pm0.38 km/s/kpc^3, (the corresponding Oort constants are A=17.9\pm0.5 km/s/kpc, B=-13.6\pm1.0 km/s/kp
:?:

I have heard about spiral density waves. I don't know if I really understand what they are, except that I think they are a product of the rotation of the galaxy and that the spiral density waves are responsible for piling up matter in such a way that we get spiral arms. But I don't understand what it means that
the spiral density wave parameters, namely: the perturbation amplitudes for the radial and azimuthal velocity components, respectively, f_R = -12.5\pm1.1 km/s and f_{\theta}=2.0\pm1.6 km/s
:?:

I understand, more or less, what the pitch angle for the spiral pattern means. It has to do with how much the spiral arms "flare out" from a bar structure, or how tightly they are "drawn towards" the bar structure. I think the pitch angle has to do with the mass of the black hole in the in the center of the galaxy, which is of course hugely interesting. Take a look at this image of NGC 5247 by Adam Block and Tom Boerner and David Young:

Image

The arms seem to "flare out" quite a lot, which suggests to me that the black hole in the center of this galaxy isn't very massive.

But take a look at this Adam Block image of NGC 1300:
The black hole seems to exert a strong pull on the spiral arms, preventing them from "flaring out".

What about the pitch angle of the Milky Way's spiral arms? How much do they "flare out? What does it mean that
the spiral density wave parameters, namely: the perturbation amplitudes for the radial and azimuthal velocity components, respectively, f_R = -12.5\pm1.1 km/s and f_{\theta}=2.0\pm1.6 km/s
:?:

And what does it mean that
the Sun's phase in the spiral wave {\chi}_o=-91\pm4 degrees.
:?:

Ann
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neufer
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The Sun's "peculiar velocity"

Post by neufer » Fri Jul 01, 2011 12:45 pm

The Sun is moving with its neighbors with a clockwise elliptical rotational speed of 252 km/second.

The Sun's "peculiar velocity" relative to its neighbors in Cartesian coordinates:
  • 1) towards the galactic center +0.6 km/s
    2) in the clockwise circular rotational direction +1.0 km/s
    3) in the (out of plane) vertical direction +0.4 km/s
(Uo, Vo, Wo)= (0.6, 1.0, 0.4) km/s

Speed of the Sun's "peculiar velocity" = sqrt(1.02 + 0.62 + 0.42) = 1.23 km/s

Galactic coordinates of the Sun's "peculiar velocity" direction = 59° 19°
(as compared with the galactic coordinates of Vega = 67.45° 19.23°)

where:

tan(59°) = 1.0 / 0.6
tan(19°) = 0.4 / sqrt(1.02 + 0.62)
http://en.wikipedia.org/wiki/Sun wrote: <<The Apex of the Sun's Way, or the solar apex, is the direction that the Sun travels through space in the Milky Way, relative to other nearby stars. The general direction of the Sun's galactic motion is towards the star Vega in the constellation of Lyra at an angle of roughly 60 sky degrees to the direction of the Galactic Center. The Sun's orbit around the Galaxy is expected to be roughly elliptical with the addition of perturbations due to the galactic spiral arms and non-uniform mass distributions. In addition the Sun oscillates up and down relative to the galactic plane approximately 2.7 times per orbit. This is very similar to how a simple harmonic oscillator works with no drag force (damping) term. It has been argued that the Sun's passage through the higher density spiral arms often coincides with mass extinctions on Earth, perhaps due to increased impact events. It takes the Solar System about 225–250 million years to complete one orbit of the galaxy (a galactic year), so it is thought to have completed 20–25 orbits during the lifetime of the Sun. The orbital speed of the Solar System about the center of the Galaxy is approximately 251 km/s. At this speed, it takes around 1,190 years for the Solar System to travel a distance of 1 light-year, or 7 days to travel 1 AU.>>
Art Neuendorffer

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