Quick little search for TW Hya moving

Michael Richmond
Apr 27, 2018

After hearing of the possible new member of the TW Hya group, I was inspired to do a quick search for additional members using the Gaia DR2 archive. Using the

• parallax
• proper motions in RA and Dec
• RA and Dec positions

SELECT *
  FROM gaiadr2.gaia_source
  WHERE parallax >= 13.8 and parallax <= 19.8
  AND ra >= 140.5 and ra <= 190.5
  AND dec >= -50.0 and dec <= -20.0
  AND pmra >= -83.0 and pmra <= -53.0
  AND pmdec >= -28.0 and pmdec <= 2.0

Running this query took a few seconds and yielded a total of 97 objects. You can find an ASCII copy of the results here:

• ASCII version of results (space-delimited)

I've made a few plots to illustrate the properties of the resulting objects. In each plot, TW Hya is indicated by a big black circle.

Position on sky.

Proper motions.

Proper motion (RA) and parallax.

Color-magnitude diagram. The numbers are the indices of the stars in the result of the query (but offset by 1 -- so the first star in the list has label "2", the second star "3", and so forth). Click on the diagram below for a high-resolution Postscript version.

Looking at this figure, we see an ordinary main sequence, with a smaller number of stars delineating a second sequence just above it. These "superdwarfs" may be particular young stars --- like TW Hya, which is clearly a member of the second group.

The labels corresponding to these stars are:

61
44
19  (TW Hya)
74
81
80
91
56
2
69
72
48
83
50
92

And thus we can identify these stars in the list returned by our query to the Gaia database: they will be the 61th, 44th, 19th, etc., stars in the list.

Let's list these stars, shall we?

# n   Gaia_source_id        RA          Dec   parallax  pmra    pmdec   Gmag   Gagne   2MASS or other
#                          (deg)       (deg)    (mas) (mas/yr) (mas/yr) 
  2 5414158429569765632   154.8231   -44.6267   14.7   -57.30   -0.61  12.521    n*    J10191763-4437360
 19 5401795662560500352   165.4659   -34.7048   16.6   -68.39  -14.02  10.439    y     TW Hya
 44 5374520249409189760   170.3407   -47.6009   14.1   -68.26  -27.02   9.980    n*    J11212188-4736028
 48 6147119548096085376   188.9536   -39.8403   14.0   -55.94  -24.83  12.963    y     TWA 11C
 50 3465989374664029184   180.6580   -33.4780   15.8   -66.25  -23.46  14.028    y     TWA 36
 56 3466308095597260032   181.8637   -32.7835   18.0   -72.78  -25.88  11.438    y     TWA 23
 61 5666748067585937408   149.7848   -22.6596   16.6   -61.04  -13.81   9.588    r     J09590842-2239345
 69 5397574190745629312   171.7136   -38.4154   14.5   -60.84  -15.51  12.961    n*    J11265133-3824553
 72 5460240959050125568   153.0377   -31.4126   18.8   -79.43  -10.54  12.714    y     J10120908-3124451
 74 5460728077057780864   154.6193   -31.8342   15.2   -56.61  -18.46  10.847    y     TWA 6A
 80 5399220743767211264   170.3223   -34.7806   16.7   -69.00  -16.87  10.904    y     J11211745-3446497
 81 5399220743767211776   170.3214   -34.7794   16.7   -69.10  -18.13  10.877    y     J11211723-3446454
 83 5378040370245563008   179.8658   -45.1721   14.1   -56.15  -18.25  13.070    h     J11592786-4510192
 91 5396105586807802880   170.2725   -38.7547   15.3   -62.90  -14.68  11.687    y     TWA 12
 92 5467714064704570112   157.1905   -28.5105   16.3   -65.23  -12.46  14.568    y     TWA 34

The column labelled "Gagne" has

• "y" if the star is listed as a member of TW Hya association in Gagne et al., ApJS 228, 18 (2017)
• "n*" if the star does not appear at all in Gagne et al., ApJS 228, 18 (2017)
• "h" if the star is listed as a high-likelihood member of TW Hya association in Gagne et al., ApJS 228, 18 (2017)
• "r" if the star is rejected as a member of TW Hya association in Gagne et al., ApJS 228, 18 (2017)

Note that 3 of these 15 stars (marked with n*) do not appear in Gagne et al. (2017). Perhaps they are new members of the TW Hya association.

Copyright © Michael Richmond. This work is licensed under a Creative Commons License.