<p>What's happening is that (i) the star is spinning and (ii) it is being eclipsed (by an exoplanet perhaps?)</p>
<p>As the exoplanet (or whatever dark object eclipses the star, the same explanation would apply to a brown dwarf or very low-mass companion) traverses across the face of the star it will eclipse parts of the stellar surface that gave their own specific line of sight velocity.</p>
<p>For example if we are seeing the system edge-on to the stellar equator and the planet orbits in prograde fashion with zero inclination, then it will first eclipse the side of the star that is spinning towards us. This takes a blue-shifted chunk out of the stellar spectrum, leading to a slight redshift (compared to what might have been expected). As the transit continues, the exoplanet moves across the centre of the star and then to a position blocking the redshifted side of the star, leading to a slight blueshift compared to what would be expected.</p>
<p>The image below (by AMitchell and shared under <a href="https://creativecommons.org/licenses/by-sa/4.0/deed.en" rel="noreferrer">CC Attribution-Share Alike 4.0 International</a> shows this situation.</p>
<p><a href="https://i.sstatic.net/HOHgV.gif" rel="noreferrer"><img src="https://i.sstatic.net/HOHgV.gif" alt="The RM effect" /></a></p>
<p>Your plot shows this happening the other way around: a blueshift followed by a redshift. This can be explained if the exoplanet orbits in retrograde fashion, so it obscures redshifted, followed by blue-shifted stellar light.</p>

CC BY-SA 4.0
2021-01-24T21:11:22.820Z
Thank you ProfRob, thats a great help.
banyans
CC BY-SA 4.0
2021-01-24T20:44:38.527Z
2021-01-24T20:44:38.527Z
ProfRob
<p>I am trying to understand the effect of the <a href="https://en.wikipedia.org/wiki/Rossiter%E2%80%93McLaughlin_effect" rel="nofollow noreferrer">Rossiter–McLaughlin effect</a> on radial velocity plots. Can someone help me and explain what exactly is happening in the red circle?</p>
<p><a href="https://i.sstatic.net/VKiHq.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/VKiHq.jpg" alt="enter image description here" /></a></p>

<observational-astronomy><astrophysics><radial-velocity>
Radial Velocity Graph and Rossiter Mclaughlin Effect
banyans
# Radial Velocity Graph and Rossiter Mclaughlin Effect

(Asked by: username_0 on 2021-01-24)

I am trying to understand the effect of the [Rossiter–McLaughlin effect](https://en.wikipedia.org/wiki/Rossiter%E2%80%93McLaughlin_effect) on radial velocity plots. Can someone help me and explain what exactly is happening in the red circle?

[![enter image description here](https://i.sstatic.net/VKiHq.jpg)](https://i.sstatic.net/VKiHq.jpg)

**Tags:** observational-astronomy, astrophysics, radial-velocity


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## 1 answers

### Answer by username_1 on 2021-01-24. Score: 6 | Accepted answer

What's happening is that (i) the star is spinning and (ii) it is being eclipsed (by an exoplanet perhaps?)

As the exoplanet (or whatever dark object eclipses the star, the same explanation would apply to a brown dwarf or very low-mass companion) traverses across the face of the star it will eclipse parts of the stellar surface that gave their own specific line of sight velocity.

For example if we are seeing the system edge-on to the stellar equator and the planet orbits in prograde fashion with zero inclination, then it will first eclipse the side of the star that is spinning towards us. This takes a blue-shifted chunk out of the stellar spectrum, leading to a slight redshift (compared to what might have been expected). As the transit continues, the exoplanet moves across the centre of the star and then to a position blocking the redshifted side of the star, leading to a slight blueshift compared to what would be expected.

The image below (by AMitchell and shared under [CC Attribution-Share Alike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/deed.en) shows this situation.

[![The RM effect](https://i.sstatic.net/HOHgV.gif)](https://i.sstatic.net/HOHgV.gif)

Your plot shows this happening the other way around: a blueshift followed by a redshift. This can be explained if the exoplanet orbits in retrograde fashion, so it obscures redshifted, followed by blue-shifted stellar light.

> **Comments:**
> **username_0**: Thank you username_1, thats a great help.  

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