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Which star has the highest known metallicity? The highest I know of is Sirius

Which a Metallicity of +0.5, which corresponds to have a Ratio of Fe to H 3 times that of the sun.

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This is a question which seems easy on first sight, but is not so easy to answer at all (I started to edit this with the intend to quickly find a value from Simbad... but this turns out to be more a non-answer and illustration how to find out with more time than I already spent searching through Simbad and its references - but I hope it might still be helpful):

Even the values for one star can scatter quite a lot throughout literature - quite surprising also for me to see. From Simbad for Sirius, the "usually" quoted metalicity of 0.5 from 2001 does not seem to be the latest measurement, but there is a newer of 0.2 from 2016. Thus generally it seems rather poorly constrained, and possibly depends on how it is measured - which will make it hard(er) to compare values from different stars if the origin of the values does not derive from the same measurement campaign. All available data for Sirius on Simbad look like this:

| Teff  log.g | Fe_H  c|CompStar |CatNo|     Reference     |
------------------------------------------------------------
|  9800 4.2   | 0.2    |SUN      |     |2016A&A...589A..83G|
|  9882 4.30  | 0.26   |SUN      |     |1989A&A...225..125L|
|  9882       | 0.40   |SUN      |     |1978ApJ...219..515B|
|  9880 4.40  | 0.50   |SUN      |     |2001ApJ...548..953Q|
|  9870 4.40  | 0.25   |SUN      |     |1991A&A...249..205B|
|  9870 4.32  | 0.12   |SUN      |     |1993A&A...276..142H|
|  9870 4.40  | 0.49   |SUN      |     |1997A&A...323..901H|
|  9824       |        |         |     |2013ApJ...771...40B|
|  9797       |        |         |     |2013ApJ...771...40B|
|  9744       |        |         |     |2013ApJ...771...40B|
|  9711       |        |         |     |2013ApJ...771...40B|
|  9707       |        |         |     |2013ApJ...771...40B|
|  9705       |        |         |     |2013ApJ...771...40B|
|  9692 4.30  | 0.87   |SUN      |     |1969A&A.....3..169G|
|  9692 4.30  | 0.81   |SUN      |     |1964ZA.....60..115K|
|  9333       | 0.70   |SUN      |     |1962ApJ...135..459W|
|  9000 4.00  | 0.71   |SUN      |     |1966ApJ...146..880S|
|  9000       | 0.00   |SUN      |     |1978ApJ...219..515B|
| 10500 3.50  | 0.40   |SUN      |     |1967cmrs..R....339A|
| 10286 4.30  | 0.82   |SUN      |     |1970SAOSR.321....1L|
| 10080 4.44  |-0.36   |SUN      |     |1966MNRAS.133..389W|
| 10080 4.30  | 0.60   |SUN      |     |1981ApJ...248.1031B|
| 10080 4.20  | 0.80   |SUN      |     |1968Obs....88..160S|
|             | 0.28   |SUN      |     |1995A&A...294..536H|
|  9580 4.20  |        |         |     |2003AJ....126.2048G|
|             | 0.28   |SUN      |F684 |1995A&A...294..536H|
|  9870 4.32  | 0.12   |SUN      |F627 |1993A&A...276..142H|
|  9870 4.40  | 0.25   |SUN      |F585 |1991A&A...249..205B|
|  9882 4.30  | 0.26   |SUN      |F554 |1989A&A...225..125L|
| 10080 4.3   | 0.60   |SUN      |F363 |1981ApJ...248.1031B|
|  9000       | 0.00   |SUN      |F310 |1978ApJ...219..515B|
|  9882       | 0.40   |SUN      |F310 |1978ApJ...219..515B|
| 10286 4.30  | 0.82   |SUN      |F150 |1970SAOSR.321....1L|
|  9692 4.3   | 0.87   |SUN      |F116 |1969A&A.....3..169G|
| 10080 4.2   | 0.80   |SUN      |F111 |1968Obs....88..160S|
| 10500 3.5   | 0.40   |SUN      |F85  |1967cmrs..R....339A|
| 10080 4.44  |-0.36   |SUN      |F83  |1966MNRAS.133..389W|
|  9000 4.0   | 0.71   |SUN      |F81  |1966ApJ...146..880S|
|  9692 4.3   | 0.81   |SUN      |F48  |1964ZA.....60..115K|
|  9333       | 0.70   |SUN      |F31  |1962ApJ...135..459W|

One can use Simbad to query for all stars with a metallicity measurement (criteria search, Fe_H.metidx > 1) and sort the resulting table by the entries in the Fe_H column. This gives a number of stars with a value of about 10: https://simbad.cds.unistra.fr/simbad/sim-sam?Criteria=Fe_H.metidx+%3E+1&submit=submit+query&OutputMode=LIST&maxObject=1000&CriteriaFile=

One of the brightest stars with a high metallicity index is according to this list $\xi$ Puppis with a [Fe/H] ~ 9.9. Yet many very high values for the metallicity seem to come from one study (Lyubimkov et al, 2019) where Simbad displays MUCH higher values for these stars than previous studies found. Even then, also for other studies one finds very high values (e.g. Luck & Lambert (2011)) - but looking at that paper it seems that the entries in the Simbad table erronously indicate the FeI/H ratio (ratio of single-ionized iron to hydrogen in the spectrum) instead of the metallicity [Fe/H]...

The value of 0.5 for Sirius is much better believable as it comes from a dedicated study of composition of Vega and Sirius.

So in essence: Sirius is one of the high - metallicity stars, yet there likely are others with higher metallicity... one would need to try compare the methods for the individual quoted studies and see which yield reliable results. But that is tedious to find the highest-known value as one really has to look at the tabular output carefully...

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