<p>The ESA <a href="http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916" rel="nofollow">states it pretty clearly</a> (although their figure of 855.2 nm is incorrect; it should be 866.2 nm):</p>

<blockquote>
  <p>The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets. For these late-type stars, the RVS wavelength interval displays, besides numerous weak lines mainly due to Fe, Si, and Mg, three strong ionised calcium lines (at around 849.8, 854.2, and 855.2 nm).</p>
</blockquote>

<p>Using <a href="https://en.wikipedia.org/wiki/Wien%27s_displacement_law" rel="nofollow">Wien's law</a>, we can see that stars with these as peak wavelengths in this interval correspond to effective temperatures in the range of 3000-3500 K:
$$T=\frac{b}{\lambda_{\text{max}}}$$
$$\begin{array}{|c|c|}
\hline \text{Wavelength (nm)}&amp;\text{Temperature (K)}\\
\hline 847 &amp; 3431\\
\hline 849.8 &amp; 3409\\
\hline 854.2 &amp; 3392\\
\hline 866.2 &amp; 3345\\
\hline 874 &amp; 3315\\
\hline
\end{array}$$
In reality, the majority of the stars Gaia studies have the most intense emissions at effective temperatures higher than this; these peaks correspond to hot M-type stars, not K- or G- type stars. The Sun, for instance, has an effective temperature of about 5800 K, and many K-type stars have effective temperatures around 4000 K. However, the target stars still guarantee intense emissions in the relevant parts of the spectrum, and thus noticeable calcium lines.</p>

CC BY-SA 3.0
2016-09-16T18:48:00.660Z
2016-09-17T03:22:12.760Z
2016-09-17T03:22:12.760Z
HDE 226868
<p>According to <a href="https://arxiv.org/abs/1010.0419">Cropper and Katz 2011</a> part 2.2,
the RVS working group considered other bands,
but the ~850 nm band is relatively unaffected by absorption in the Earth's atmosphere,
facilitating ground-based preparation and follow-up.
In addition to the strong Ca II triplet,
this band is rich in lines enabling study of astrophysical quantities other than radial velocity,
adding to the science return on the spectrometer investment.</p>

<p>For type B and hotter stars, a small minority of the population,
they hope to get radial velocity from
the Paschen <a href="https://en.wikipedia.org/wiki/Hydrogen_spectral_series">hydrogen series</a>,
which accounts for the wide troughs at 854.3, 859.6, and 866.3 nm in the top of
<a href="http://cds.cern.ch/record/531022/files/0109057.pdf">Munari 2001</a> figure 2.</p>

CC BY-SA 3.0
2016-09-17T01:13:14.620Z
Thanks - this is very helpful to understand better the various considerations involved in selecting the final wavelength band for the RVS.
uhoh
CC BY-SA 3.0
2016-09-16T19:59:40.407Z
2016-09-16T21:17:02.320Z
2016-09-16T21:17:02.320Z
Mike G
<p>The Ca triplet in the near infrared are extremely strong resonance <em>absorption</em> lines. They are by far the strongest features in the near infrared spectra of cool G,K,M type dwarfs and giants, which will be the majority of the stars observed by the Gaia RVS.
The Ca triplet lines are so strong that even in low metallicity halo stars, that have little Ca in their photospheres, these lines are <em>still</em> strong enough to measure radial velocities.</p>

<p>The lines are much weaker and much broader for hotter O, B and A stars, and measuring radial velocities for these will be difficult and much less precise.</p>

<p>You can have a look at an atlas of the Gaia Ca triplet region for stars of  different spectral types in Figure 2 of Munari et al. (2001). <a href="http://cds.cern.ch/record/531022/files/0109057.pdf" rel="nofollow">http://cds.cern.ch/record/531022/files/0109057.pdf</a></p>

<p>I should also add that these three lines are <em>not</em> the only features used to determine the velocities, they are just the strongest features in the spectra of most stars.</p>

CC BY-SA 3.0
2016-09-16T19:08:55.350Z
2016-09-17T06:43:02.323Z
2016-09-17T06:43:02.323Z
ProfRob
<p>I was reading <a href="http://spaceflight101.com/gaia/gaia-instrument-information/" rel="nofollow noreferrer">this overview article</a> about the Gaia spacecraft and I saw the following statement:</p>
<blockquote>
<p>These spectra provide radial velocity information that are used to study the kinematic and dynamic evolution of the Milky Way. Radial velocities are derived from three isolated Calcium lines at 849.8, 854.2 and 855.2nm. Other lines in the 847 to 874nm range can provide data on star composition, surface gravity, and metal abundance.</p>
</blockquote>
<p><strong>note:</strong> as pointed out in the comments below, the third line is at <em>866.2nm, not 855.2nm</em> - this is suggested to be a typo - the '855' number also shows up on <a href="http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916" rel="nofollow noreferrer">this ESA page</a>.</p>
<p>The high resolution spectroscopy only seems to operate between 847 and 874nm, and &quot;three isolated Calcium lines&quot; are used to measure radial velocity.</p>
<p>Do all stars have enough calcium in their atmosphere to produce strong enough features to measure the radial velocity so accurately? I had thought that there are some stellar populations have very little besides hydrogen and helium in their atmosphere.</p>
<p>Are these always emission lines or absorption lines, or will there be some stars with one and some of the other? What fraction of stars simply won't have significant amounts of calcium?</p>
<p><a href="https://i.sstatic.net/GS4hZ.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/GS4hZ.png" alt="Gaia's Radial Velocity Spectrometer (ESA)" /></a></p>
<p><strong>above:</strong> Gaia's Radial Velocity Spectrometer from <a href="http://spaceflight101.com/gaia/gaia-instrument-information/" rel="nofollow noreferrer">here</a>, credit: ESA.</p>
<p><a href="https://i.sstatic.net/A5eLx.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/A5eLx.jpg" alt="Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array" /></a></p>
<p><strong>above:</strong> Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array, from <a href="http://spaceflight101.com/gaia/gaia-instrument-information/" rel="nofollow noreferrer">here</a>, credit: EADS Astrium.</p>
<p><a href="https://i.sstatic.net/jVhdY.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/jVhdY.jpg" alt="Gaia's Optical Module, including Radial Velocity spectrometer (gratings) and a focal field corrector; SAS Astrium" /></a></p>
<p><strong>above:</strong> Gaia's Optical Module, including Radial Velocity spectrometer (gratings) and a focal field corrector, from <a href="http://spaceflight101.com/gaia/gaia-instrument-information/" rel="nofollow noreferrer">here</a>, credit: SAS Astrium.</p>

CC BY-SA 3.0
2016-09-16T21:16:16.677Z
[Other sources](https://en.wikipedia.org/wiki/Calcium_triplet) have the third Ca II line at 866.2 nm, not 855.2 nm.
Mike G
CC BY-SA 3.0
2016-09-17T00:00:59.577Z
Correct, it's a typo. 866.2 nm.
ProfRob
CC BY-SA 3.0
2016-09-17T00:48:58.930Z
@RobJeffries the '855' value shows up on [this ESA page](http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916) as well, as shown in [this answer](http://astronomy.stackexchange.com/a/18376/7982) below. I've added a note in the question (I don't want to help propagate the number if it's incorrect). I wonder how far it goes! A quick google search shows [849.8 nm,  855.2 nm and 866.2 nm](https://books.google.com.tw/books?id=avns1CRdaCYC&lpg=PA385&ots=icPC_KYvU1&dq=calcium%20II%20855.2&pg=PA385#v=onepage&q=calcium%20II%20855.2&f=false) which contains '855' in a different location.
uhoh
CC BY-SA 3.0
2016-09-17T05:38:05.670Z
In order to accommodate a very large number of stellar spectra simultaneously superimposed on the RVS CCD array at high dispersion, it seems to have been necessary to choose only a narrow wavelength range. So far there are three good answers here that explain that the Ca II triplet is present in a wide range of stars and usually narrow, the Paschen hydrogen series is close by for the hotter stars, and it is near the "energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets." In this case I can't choose a single "accepted" answer & encourage up-voting them all!
uhoh
CC BY-SA 3.0
2016-09-17T06:40:26.253Z
The Ca IR triplet is at 849.8, 854.2 and 866.2 nm https://en.m.wikipedia.org/wiki/Calcium_triplet See also any picture of a spectrum!
ProfRob
CC BY-SA 3.0
2016-09-17T06:59:17.180Z
@RobJeffries indeed! I was just looking around to see how widespread the use of the '855' value was, since it's wrong on in the ESA article, and noted how easy it was to find other errors. Astronomers are usually good at getting things right - I was surprised to see the "propagation of an error" so to speak.
uhoh
<spectroscopy><velocity><stellar-atmospheres><metallicity><gaia>
Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?
uhoh
# Why does Gaia use only calcium NIR lines for stellar radial velocity measurements?

(Asked by: username_0 on 2016-09-16)

I was reading [this overview article](http://spaceflight101.com/gaia/gaia-instrument-information/) about the Gaia spacecraft and I saw the following statement:

> These spectra provide radial velocity information that are used to study the kinematic and dynamic evolution of the Milky Way. Radial velocities are derived from three isolated Calcium lines at 849.8, 854.2 and 855.2nm. Other lines in the 847 to 874nm range can provide data on star composition, surface gravity, and metal abundance.

**note:** as pointed out in the comments below, the third line is at *866.2nm, not 855.2nm* - this is suggested to be a typo - the '855' number also shows up on [this ESA page](http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916).

The high resolution spectroscopy only seems to operate between 847 and 874nm, and "three isolated Calcium lines" are used to measure radial velocity.

Do all stars have enough calcium in their atmosphere to produce strong enough features to measure the radial velocity so accurately? I had thought that there are some stellar populations have very little besides hydrogen and helium in their atmosphere.

Are these always emission lines or absorption lines, or will there be some stars with one and some of the other? What fraction of stars simply won't have significant amounts of calcium?

[![Gaia's Radial Velocity Spectrometer (ESA)](https://i.sstatic.net/GS4hZ.png)](https://i.sstatic.net/GS4hZ.png)

**above:** Gaia's Radial Velocity Spectrometer from [here](http://spaceflight101.com/gaia/gaia-instrument-information/), credit: ESA.

[![Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array](https://i.sstatic.net/A5eLx.jpg)](https://i.sstatic.net/A5eLx.jpg)

**above:** Gaia's Imaging system, including mirrors 4, 5 and 6, prisms, diffraction gratings, and CCD array, from [here](http://spaceflight101.com/gaia/gaia-instrument-information/), credit: EADS Astrium.

[![Gaia's Optical Module, including Radial Velocity spectrometer (gratings) and a focal field corrector; SAS Astrium](https://i.sstatic.net/jVhdY.jpg)](https://i.sstatic.net/jVhdY.jpg)

**above:** Gaia's Optical Module, including Radial Velocity spectrometer (gratings) and a focal field corrector, from [here](http://spaceflight101.com/gaia/gaia-instrument-information/), credit: SAS Astrium.

**Tags:** spectroscopy, velocity, stellar-atmospheres, metallicity, gaia
> **Comments:**
> **username_1**: [Other sources](https://en.wikipedia.org/wiki/Calcium_triplet) have the third Ca II line at 866.2 nm, not 855.2 nm.  
> **username_2**: Correct, it's a typo. 866.2 nm.  
> **username_0**: @RobJeffries the '855' value shows up on [this ESA page](http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916) as well, as shown in [this answer](http://astronomy.stackexchange.com/a/18376/7982) below. I've added a note in the question (I don't want to help propagate the number if it's incorrect). I wonder how far it goes! A quick google search shows [849.8 nm,  855.2 nm and 866.2 nm](https://books.google.com.tw/books?id=avns1CRdaCYC&lpg=PA385&ots=icPC_KYvU1&dq=calcium%20II%20855.2&pg=PA385#v=onepage&q=calcium%20II%20855.2&f=false) which contains '855' in a different location.  
> **username_0**: In order to accommodate a very large number of stellar spectra simultaneously superimposed on the RVS CCD array at high dispersion, it seems to have been necessary to choose only a narrow wavelength range. So far there are three good answers here that explain that the Ca II triplet is present in a wide range of stars and usually narrow, the Paschen hydrogen series is close by for the hotter stars, and it is near the "energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets." In this case I can't choose a single "accepted" answer & encourage up-voting them all!  
> **username_2**: The Ca IR triplet is at 849.8, 854.2 and 866.2 nm https://en.m.wikipedia.org/wiki/Calcium_triplet See also any picture of a spectrum!  
> **username_0**: @RobJeffries indeed! I was just looking around to see how widespread the use of the '855' value was, since it's wrong on in the ESA article, and noted how easy it was to find other errors. Astronomers are usually good at getting things right - I was surprised to see the "propagation of an error" so to speak.  

---

## 3 answers

### Answer by username_2 on 2016-09-16. Score: 11 | Accepted answer

The Ca triplet in the near infrared are extremely strong resonance *absorption* lines. They are by far the strongest features in the near infrared spectra of cool G,K,M type dwarfs and giants, which will be the majority of the stars observed by the Gaia RVS.
The Ca triplet lines are so strong that even in low metallicity halo stars, that have little Ca in their photospheres, these lines are *still* strong enough to measure radial velocities.

The lines are much weaker and much broader for hotter O, B and A stars, and measuring radial velocities for these will be difficult and much less precise.

You can have a look at an atlas of the Gaia Ca triplet region for stars of different spectral types in Figure 2 of Munari et al. (2001). <http://cds.cern.ch/record/531022/files/0109057.pdf>

I should also add that these three lines are *not* the only features used to determine the velocities, they are just the strongest features in the spectra of most stars.


---

### Answer by username_3 on 2016-09-16. Score: 7

The ESA [states it pretty clearly](http://sci.esa.int/gaia/40129-payload-module/?fbodylongid=1916) (although their figure of 855.2 nm is incorrect; it should be 866.2 nm):

> The RVS wavelength range, 847-874 nm, has been selected to coincide with the energy-distribution peaks of G- and K-type stars which are the most abundant RVS targets. For these late-type stars, the RVS wavelength interval displays, besides numerous weak lines mainly due to Fe, Si, and Mg, three strong ionised calcium lines (at around 849.8, 854.2, and 855.2 nm).

Using [Wien's law](https://en.wikipedia.org/wiki/Wien%27s_displacement_law), we can see that stars with these as peak wavelengths in this interval correspond to effective temperatures in the range of 3000-3500 K:
$$T=\frac{b}{\lambda\_{\text{max}}}$$
$$\begin{array}{|c|c|}
\hline \text{Wavelength (nm)}&\text{Temperature (K)}\\
\hline 847 & 3431\\
\hline 849.8 & 3409\\
\hline 854.2 & 3392\\
\hline 866.2 & 3345\\
\hline 874 & 3315\\
\hline
\end{array}$$
In reality, the majority of the stars Gaia studies have the most intense emissions at effective temperatures higher than this; these peaks correspond to hot M-type stars, not K- or G- type stars. The Sun, for instance, has an effective temperature of about 5800 K, and many K-type stars have effective temperatures around 4000 K. However, the target stars still guarantee intense emissions in the relevant parts of the spectrum, and thus noticeable calcium lines.


---

### Answer by username_1 on 2016-09-16. Score: 6

According to [Cropper and Katz 2011](https://arxiv.org/abs/1010.0419) part 2.2,
the RVS working group considered other bands,
but the ~850 nm band is relatively unaffected by absorption in the Earth's atmosphere,
facilitating ground-based preparation and follow-up.
In addition to the strong Ca II triplet,
this band is rich in lines enabling study of astrophysical quantities other than radial velocity,
adding to the science return on the spectrometer investment.

For type B and hotter stars, a small minority of the population,
they hope to get radial velocity from
the Paschen [hydrogen series](https://en.wikipedia.org/wiki/Hydrogen_spectral_series),
which accounts for the wide troughs at 854.3, 859.6, and 866.3 nm in the top of
[Munari 2001](http://cds.cern.ch/record/531022/files/0109057.pdf) figure 2.

> **Comments:**
> **username_0**: Thanks - this is very helpful to understand better the various considerations involved in selecting the final wavelength band for the RVS.  

---

