A GALEX Far-ultraviolet Study of Dwarf Stars in Moving Groups

In the field of stellar kinematics, the term "moving group" has come to be used for an ensemble of stars with similar space velocities that are thought to have originated together but are not currently bound within a cluster. Although the concept dates back more than a century, the detailed study of moving groups was pioneered by Eggen (1965a) who identified a number of such systems (Eggen 1996). Some groups have velocities that are similar to those of known star clusters such as the Hyades, such that the group stars may be escapees from said cluster (Eggen 1970b), but this is not always the case.

Assigning stars to a group on the basis of similar space motion alone is not sufficient to prove that these stars originally formed together in a cluster or association. Boyle & McClure (1975), for example, measured DDO photometry for stars suggested by Eggen to be members of the Hyades group, and found that some candidates did not follow the DDO color–color diagrams of stars in the Hyades cluster. It has since become customary to use chemical composition as an additional criterion for group membership upon the assumption that stars having a common origin in time and space within star-forming clusters would have near-identical chemical compositions, as seems to be the case for open clusters (De Silva et al. 2009; Bovy 2016). Abundance studies, e.g., De Silva et al. (2007, 2011), Liu et al. (2012), Tabernero et al. (2017), and Dopcke et al. (2019), have typically revealed that not all kinematic candidates of a proposed moving group have the same metal abundances or element ratios, nonetheless in most cases a sample of chemically homogeneous stars can be identified. Thus abundance studies have confirmed the concept of kinematically and chemically homogeneous moving groups of field stars.

Kinematic groups can therefore offer samples of potentially coeval stars that are closer than most Milky Way open clusters. As such, moving groups lend themselves to the study of a number of astrophysical phenomena, particularly among dwarf stars, whose numbers within such groups tend to exceed those of the more luminous giant stars. One such phenomenon is the evolution of stellar activity among main-sequence stars of spectral type F through M, the study of which can be facilitated by a comparison of chromospheric and coronal properties among dwarfs associated with moving groups of different age. Soderblom & Mayor (1993a, 1993b) and Mamajek & Hillenbrand (2008), for example, have used this approach to document the age dependence of Ca ii H and K emission among FGK dwarfs. Other proxies for stellar activity among late-type dwarfs can also be investigated with moving groups.

In the present paper, we consider an activity proxy for late-type dwarf stars that is based upon GALEX far-ultraviolet (FUV) photometry. Stars chosen for this study have at one time or another been suggested for membership in a moving group. The kinematic systems considered here are the Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, Kapteyn's, AB Doradus, ζ Herculis, σ Puppis, η Cephei, β Pictoris, and Tucana–Horologium moving groups. An activity proxy is utilized here that is based on the position of a star within a two-color diagram that combines Johnson B and V photometry with FUV magnitude data from the GALEX space telescope. The GALEX observatory was active from 2003 through 2013. A primary objective of the mission was to image galaxies in both near-ultraviolet (NUV) and FUV wavelength bands, so as to enable studies of extragalactic star formation (Martin et al. 2005). Over the course of its mission, GALEX imaged much of the sky in the NUV and FUV bands, giving a legacy of broadband photometry not only of other galaxies but also Galactic stars at wavelengths shortward of the Earth's atmospheric cut-off (Bianchi 2014).

At FUV wavelengths the brightness of F, G, and K dwarfs has been found to correlate with the level of chromospheric activity (e.g., Smith & Redenbaugh 2010; Findeisen et al. 2011; Smith et al. 2017; Richey-Yowell et al. 2019), which in turn is known to be age dependent (Duncan et al. 1991; Soderblom 2010). Samples of coeval stars might therefore be expected to display well-defined loci in photometric two-color diagrams that incorporate an FUV magnitude in one of the colors. Smith (2018a) showed this to be the case for main-sequence stars in several open clusters when plots are made of a GALEX-based (FUV − B) color versus B − V. In this paper we present an analogous study of stars that have been proposed for membership in a number of kinematic groups. One benefit of such groups is that they can yield samples of stars that are closer and brighter than is typical of open clusters.

The study of FUV activity among moving group stars has been pioneered by a number of authors. Findeisen et al. (2011) used several moving groups in their study of stellar activity in the broadband ultraviolet. Murgas et al. (2013) used a pair of stellar activity indicators, Ca ii H and K emission and an (FUV − V) color, for tracing the membership of moving groups. Rodriguez et al. (2013) exploited GALEX photometry to assess the membership of M stars in several young associations and kinematic groups. A study of FUV activity specifically among K stars in moving groups using GALEX data is that of Richey-Yowell et al. (2019), who based their analysis on FUV flux data. By contrast GALEX FUV magnitudes rather than fluxes are employed here, and a wider range of spectral types is considered.

The structure of this paper is as follows. In Section 2 the data to be used are introduced and discussed. Plots of color–magnitude diagrams for the stellar candidates in each moving group are used to ensure that all stars chosen for this paper are main-sequence stars. Plots of an (FUV − B) color versus B − V for dwarf stars in each group are considered in Section 3. A methodology for fitting polynomials to the main-sequence locus in this two-color diagram is discussed, and comparisons are made between the various groups. In Section 4 the two-color fits are considered in the context of age differences between groups, and a relationship is developed between a main-sequence FUV-color parameter and the age of a moving group. Section 5 summarizes the conclusions reached and possibilities for future research.

Photometric and parallax data were compiled for candidate dwarf star members of twelve moving groups. Attention was restricted to kinematic groups for which more than 20 member stars had been documented or suggested in the literature. The following groups were considered.

The Hyades moving group is a kinematic system of stars with space motions that are similar to that of the Hyades open cluster. A kinematic association of stars with velocities similar to the Hyades was first proposed by Proctor (1869). Since that time the Hyades group has been extensively studied, e.g., Eggen (1960, 1970a), Boyle & McClure (1975), Boesgaard & Budge (1988), and Mendez et al. (1992), Garcia Lopez et al. (1993). A list of Hyades group candidates has been compiled mainly from a series of papers by Eggen (1985a, 1985b, 1985c), with some additional stars from Boyle & McClure (1975).

Stars of the HR 1614 moving group have similar space motions to the dwarf HR 1614. This kinematic association was proposed by Eggen (1971a), and since that time some interestingly high abundances have been discovered among candidates for group membership by Eggen (1978a, 1978b, 1992, 1998), Smith (1983), Feltzing & Holmberg (2000), and De Silva et al. (2007). A list of stellar candidates for the HR 1614 group was compiled from papers by Eggen (1996, 1998) and Feltzing & Holmberg (2000).

The Wolf 630 moving group is associated with the star Wolf 630, and was first proposed by Eggen (1965b). A list of stars that have been assigned to this group was taken mainly from the paper of McDonald & Hearnshaw (1983), with two additional stars from Bubar & King (2010).

The Ursa Major moving group is named such because it includes many of the stars in the Ursa Major asterism. Like the Hyades moving group, it was first proposed by Proctor (1869). A sample of stars in the Ursa Major group was chosen from Soderblom & Mayor (1993a) and King et al. (2003), who categorized stars by a likelihood of group membership. Stars that they classified as "probably not members" are not considered here.

The Arcturus and Kapteyn's Star moving groups are kinematically associated with Arcturus and Kapteyn's star, respectively. They were both first proposed by Eggen (1962, 1971b). Eggen (1996) served as the source for candidate members.

A kinematic group comprising stars with space motions similar to AB Doradus was first proposed by Zuckerman & Song (2004). Multiple studies have suggested that this group may be associated with the Pleiades open cluster (Ortega et al. 2007). The sample of stars considered here for the AB Doradus group was drawn from Zuckerman & Song (2004).

The ζ Herculis, σ Puppis, and η Cephei moving groups are associated with eponymous stars. All three were proposed by Eggen (1958, 1971c), ζ Herculis in 1958 and σ Puppis and η Cephei both in 1971. Lists of stars from all three of these groups have been gleaned from the paper by Eggen (1971c).

Stars with space velocities similar to that of β Pictoris were first discussed by Barrado y Navascués et al. (1999). The present collection of stars in the β Pictoris moving group was obtained from Lee & Song (2018).

The Tucana–Horologium moving group was originally proposed as two moving groups named for the constellations Tucana and Horologium (Kraus et al. 2014). The Tucana association was first proposed by Zuckerman & Webb (2000), and the Horologium association was proposed by Torres et al. (2000). These two associations were later found to be part of a single larger moving group. The paper by Kraus et al. (2014) identifies the group members that are considered herein.

For the initial listings of moving group stars the General Catalog of Photometric Data (GCPD; Mermilliod et al. 1997) was searched for V and B − V photometry. In cases of binary systems for which there is photometry available for both component stars, i.e., in cases where the GCPD or SIMBAD data bases listed photometry for multiple component stars under a single HD/BD/other number, the data for the brighter member was chosen. Additional information was selected from the University of Strasbourg's SIMBAD database (Wenger et al. 2000). Some of the group candidates were without BV photometry in the GCPD, or have designations that the GCPD could not parse. For these stars magnitude data from SIMBAD was collected instead. Stars for which Johnson photometry could not be located in either database were dropped from further consideration.³ Initial compilations of group candidates were not limited by spectral type. Quite a few of the stars included are spectral type M and a few are type A, although FGK spectral types comprise the bulk of these samples.

Notes were taken of the following object classifications as provided by SIMBAD: double star (D), spectroscopic binary (SB), symbiotic binary (Sy), Algol-type eclipsing binary (Alg), flare star (Fl), T Tauri star (TT), chemically peculiar star (CP), variable star (V), rotationally variable star (RV), eruptive variable (Er), δ Scuti variable (δS), RS Canum Venaticorum variable (RSC), BY Draconis variable (BYD), β Lyrae class star (βL), W Ursae Majoris type variable (WU), R Coronae Borealis variable (RCB), and SX Phoenicis variable (SXP).

With initial lists of group stars in hand a variety of ancillary data was searched for, including measurements of far-ultraviolet magnitude from the GALEX space telescope. SIMBAD was the source for stellar parallax π measurements. In the great majority of cases SIMBAD listed the Gaia parallax measurement (Gaia Collaboration 2016, 2018), although for some stars the measurements came from the Hipparcos satellite (Perryman et al. 1997). Absolute visual magnitudes (M_V) were calculated. Stars for which there was no parallax data were removed from the sample.

The Mikulski Archive for Space Telescopes was used to compile FUV magnitude data measured from GALEX sky images. The GALEX FUV magnitude is based on a filter with a wavelength band of 135–175 nm (Martin et al. 2005; Morrissey et al. 2007). The Data Release 6/7 was searched, with SIMBAD being used as the name resolver, along with a search radius of 02. With these search parameters, FUV magnitude measurements were obtained for ∼50% of the stars in the initial samples compiled for this project. In the case of stars for which more than one FUV mag measurement was listed in GR6/7, the mean value of the measurements was adopted. We have elected to leave the apparent FUV magnitudes employed in this paper on the established GALEX GR6/7 AB system, and have not applied the corrections offered by Camarota & Holberg (2014).⁴ The FUV and B magnitude data compiled for this paper were used to calculate a color index that is designated herein as (FUV − B).

Measurement errors in the GALEX photometry (Morrissey et al. 2007) are typically greater than in the Johnson photometry from the GCPD (Mermilliod et al. 1997). The uncertainties in the FUV magnitudes from GALEX GR6/7 are taken to be the dominant source of error in the (FUV − B) color. Only single FUV measurements were available for many group dwarfs. However for some stars two or more magnitude measurements were given in the GR6/7 data release. Such stars served as a basis for assessing an observational uncertainty in the FUV magnitude. For each of these stars the range among the multiple values of FUV mag was used to estimate a standard deviation σ according to the small-sample statistics formalism of Wan et al. (2014). Averaging these estimates of σ, weighted by the number of measurements per star, gave a result of σ(mean) = 0.172, which is taken to be a representative uncertainty for the (FUV − B) color indices used in this paper. Some variable stars and flare stars had relatively large ranges in the FUV magnitude. These were not included in deriving the mean σ value.

Photometric Hertzsprung–Russell diagrams in the form of M_V versus B − V color index were plotted for the initial lists of stars in each group. These color–magnitude diagrams were used to exclude giant stars from further consideration. Data compiled for this paper are presented in Tables 1–12. Only dwarf stars for which GALEX FUV observations were available are listed. The AB Doradus and β Pictoris group samples show no clear giant branch, so all stars compiled for these groups were treated as dwarfs.

Four of the most populous moving groups considered here are those associated with Wolf 630, HR 1614, Ursa Major, and the Hyades. Color–magnitude diagrams of the dwarf stars with FUV data for these four groups are shown in Figures 1 and 2, with solid lines showing the zero-age main-sequence locus of VandenBerg & Poll (1989). Differences in the main-sequence turn-off extension among these groups are indicative of the age differences between them. Most of the absolute magnitudes plotted in Figures 1 and 2 are based on Gaia parallaxes, which post-date the papers that have been used as our sources for group candidates. The main-sequence CMD locus for groups such as the Hyades and Ursa Major is tighter than for other groups such as Wolf 630. On this basis it would not be surprising if some of the group listings compiled for this work have a larger fraction of non-members than the Hyades or Ursa Major samples. It is expected that Gaia parallaxes will provide new insights into the reality and membership of moving groups. However, we have not attempted to use the Gaia parallaxes to re-assess group membership for stars listed in Tables 1–12. The questions addressed in this paper are instead based on a study of stellar ensembles for each group.

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For each moving group the (FUV − B) color index has been plotted versus B − V for dwarf stars. These color–color diagrams are shown in Figures 3–14. The Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, Kapteyn's Star, AB Doradus, η Cephei, β Pictoris, and Tucana–Horologium groups all display clear loci in these two-color diagrams. The ζ Herculis group dwarfs form a locus that relies heavily on two data points, although it has a similar overall shape to the main sequence of other groups. The candidate dwarfs for the σ Puppis group do not exhibit a distinguishable locus.

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A least-squares polynomial fit to the color–color data was made for the dwarf stars in each of the eleven groups for which well-defined loci could be identified in Figures 3–14. Points that had high residuals were removed from the best-fit curves, starting with the point having the highest residual. Tables 1–12 include a column that notes whether or not each star was removed from the least-squares fits. For each group the fitting process started with a second degree polynomial, and the degree was increased up to a maximum of fifth order until either the residuals did not decrease significantly or the fit became excessively influenced by individual stars. The general best-fit equation can be written as

$$\begin{eqnarray}&&(\mathrm{FUV}-B)=\displaystyle \sum _{n=0}^{5}{a}_{n}{\left(B-V\right)}^{n},\end{eqnarray} \tag{ 1 }$$

where a_n are the coefficients of the best-fit curves that are listed in Table 13. Also tabulated is the B − V domain over which each best-fit is applicable, and the coefficient of determination (R²) of each fit. Removal from the fitting process does not necessarily imply that an individual star is not a group member; many of the stars excluded from the fits, for example, were flare stars, which exhibit temporary increases in brightness.

The loci that the group dwarfs form in these color–color diagrams show several similar features. The plots for the Hyades, Wolf 630, and Ursa Major main-sequence stars provide the best examples, as they have the widest ranges of B − V color over which the highest-degree fits are applicable. The fitted loci for these three groups each show a clear maximum value in the (FUV − B) color at B − V ∼ 0.88 (0.891, 0.871, and 0.874, respectively), along with inflection points in the fits in the vicinity of B − V ∼ 0.4 (0.404, 0.446, and 0.305, respectively).

Fits made to the two-color diagrams of dwarfs in the other moving groups do not encompass a sufficiently wide range in B − V to show both the maximum and the inflection found for the Hyades, although the fits are not inconsistent with such features. The group based on Kapteyn's star is the only other one with a main sequence that extends to B − V values bluer than the inflection point, and the polynomial fit for it forms a concave-up parabola. Seven other loci are concave-down. Five of these have maxima near the same B − V value as the Hyades, Wolf 630 and Ursa Major groups. Main-sequence stars from the β Pictoris and Tucana–Horologium groups are predominantly redder than B − V = 0.88, such that the fits in these cases do not exhibit clear maxima.

The overall shapes of the two-color loci are tied in part to the stellar temperatures. Assuming the light from a stellar photosphere to approximate a blackbody, a hotter dwarf would emit a higher proportion of light at FUV wavelengths than a cooler dwarf, in which case the (FUV − B) color would increase as B − V increases and effective temperature decreases. An increasing (FUV − B) is seen among dwarf stars at B − V < 0.88. Redder than this B − V color, however, the flux ratio (F_FUV/F_B) is observed to increase as effective temperature decreases, which is not expected for a late-type stellar photosphere. This redward behavior is instead assumed to be the consequence of FUV light contributed from a stellar chromosphere and transition region.

Figure 15 shows the color–color polynomial fits overlaid for comparison. Though the loci of dwarfs in different groups follow the same overall shape, there are clear offsets in (FUV − B) at a fixed B − V even among dwarfs with B − V < 0.88, which are attributed here to differences in the amount of FUV light being radiated from a chromosphere and transition region. The best fits of the different groups appear to converge in (FUV − B) toward hotter B − V colors and diverge with redder B − V. Compared to the cooler dwarfs in the figures the photospheres of the hotter dwarfs emit a greater proportion of light in the FUV band. The increasing contribution of photospheric FUV light toward hotter main-sequence temperatures serves to reduce the relative contribution of chromospheric and transition region emission to the FUV magnitude. Differences in FUV magnitude due to differences in stellar activity become more detectable as B − V progresses toward cooler dwarfs.

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In this section we address the supposition that the (FUV − B) color of late-type dwarfs is dependent on stellar age by virtue of the contributions of light from a chromosphere and transition region to their FUV spectra. For this purpose age data are needed for the moving groups considered here. Some of these groups are named for a specific member star, and for such groups, except the Wolf 630 and σ Puppis systems, age estimates have been compiled from the literature for the eponymous star. For the Ursa Major group an age estimate for Sirius, which is thought to be a member of that group, is adopted. For the Hyades moving group, an age estimate for the Hyades star cluster is used, because the group stars are thought to have originated from that cluster. For the Wolf 630 and Tucana–Horologium groups various age estimates were obtained from the literature. The only age estimate found for the σ Puppis group was 15 Gyr (Eggen 1970a). Because this is greater than the Hubble time, no age is assigned here to this group. The age data that have been obtained from the literature are presented in Table 14, along with [Fe/H] abundance measurements, and the applicable sources. When either multiple age and/or metallicity measurements were found for an eponymous star or kinematic group, the most recent values from the literature have been tabulated here.

To further analyze the differences between the main-sequence loci of moving groups, the best-fit equations for each group that are detailed in Table 13 were used to calculate representative values of the (FUV − B) color index at various values of B − V (within the applicable B − V range over which each curve has been fitted). Interpolated values of (FUV − B) are presented in Table 15. The lack of an entry in the table indicates that a given value of B − V was outside the range of reliability of the relevant best fit.

Interpolated values of (FUV − B) for each group are plotted against group age in Figures 16 through 24 for a number of values of the B − V color index. These plots reveal logarithmic relationships that have been fitted with least-squares lines of the form

$$\begin{eqnarray}&&(\mathrm{FUV}-B)={c}_{0}+{c}_{1}{\mathrm{log}}_{10}(\mathrm{Age}/\mathrm{Gyr}).\end{eqnarray} \tag{ 2 }$$

The coefficients of the derived fits are presented in Table 16, along with the R² coefficient of determination. Among the hotter dwarfs with B − V < 0.5 the logarithmic relationship becomes less distinct, and it disappears entirely by B − V = 0.4. This is consistent with the observation that the (FUV − B) versus B − V curves converge at warmer effective temperatures, due to increasing relative contribution of photospheric light to the FUV spectrum. At the highest temperatures among the dwarfs in Figures 3–24, the range in (FUV − B) between the groups is on the order of the observational error expected of this color index (Section 2).

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The correlations evinced by Figures 16–24 indicate that the (FUV − B) color among FGK dwarfs could potentially be calibrated for age determination purposes. The well-defined loci found in the FUV two-color diagram are also consistent with a supposition that most of the Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, AB Doradus, η Cephei, β Pictoris, and Tucana–Horologium dwarfs considered in this paper come from samples of coeval stars. Though the locus formed by the dwarf stars of the ζ Herculis group is less clear, the projected (FUV − B) values from the best fit are consistent with the FUV–age relation defined by the other groups. Although dwarfs from the Kapteyn's Star group evince a reasonably well-defined locus in the two-color diagram, the majority of stars in the sample considered here have effective temperatures for which the FUV band is dominated by light from the photosphere. The (FUV − B) sensitivity to stellar activity is reduced at the B − V colors of these stars (Smith & Redenbaugh 2010; Smith et al. 2017), such that the locus seen in Figure 8 is strongly influenced by the photospheric spectrum.

Correlations exist between values of the best-fit coefficients listed in Table 16 and B − V color. Plotted in Figures 25 and 26 are the values of c₀ and c₁, respectively, computed for 1100 values of B − V between 0.5 and 1.6. Derived values of both coefficients, especially the slope c₁, are sensitive to the number of data points used to calculate them. Discontinuities occur in Figures 25 and 26 at values of B − V that correspond to endpoints in the B − V domain specified in Table 13 for each group. For example, discontinuities are prominent at B − V = 1.0 (where the color range for the β Pictoris group starts and that for the ζ Herculis group ends), 1.1 (a color limit for the Tucana–Horologium group dwarfs), 1.3 (a color limit for the Wolf 630 sample), and 1.4 (the red limit for stars considered in the Ursa Major and AB Doradus groups). Other discontinuities result where the fits are constrained by few data points. In the case of the c₀ coefficient versus B − V it was possible to calculate a least-squares polynomial fit throughout the range 0.5 < B − V < 1.5. A third-degree fit given by the equation

$$\begin{eqnarray}&&{c}_{0}=6.58{\left(B-V\right)}^{3}-31.4{\left(B-V\right)}^{2}+40.4(B-V)-3.42,\end{eqnarray} \tag{ 3 }$$

was found to work best. The β Pictoris and Tucana–Horologium groups are the youngest in the sample and have considerable leverage on the best-fit value of c₁. In part because of this the color dependence of the c₁ coefficient is more complex and makes a polynomial fit less useful.

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A logarithmic relationship has been found between the ages of dwarf stars having a spectral type later than mid-F and a photometric index derived from a (FUV − B) color. Among such dwarf stars the chromosphere and transition region can contribute a significant fraction of the flux at far-ultraviolet wavelengths. The samples of main sequence stars considered here for the Hyades, HR 1614, Wolf 630, Ursa Major, Arcturus, AB Doradus, η Cephei, β Pictoris, and Tucana–Horologium moving groups form clear and distinct loci in the (FUV − B, B − V) two-color diagram. The samples considered here for each of these groups would appear to contain a substantial fraction of coeval stars. The main sequence stars in the ζ Herculis kinematic group evince a two-color diagram that is consistent with estimates of the age of the eponymous star, although the sample is smaller than for most other groups considered. There are too few stars with GALEX FUV data in the Kapteyn's Star and σ Puppis samples to permit a comparable conclusion for these two groups.

Correlations established in Section 4 indicate that the location of a late-type dwarf in the (FUV − B, B − V) two-color diagram could potentially be calibrated in such a way as to provide stellar age information. The logarithmic relationship between (FUV − B) color and age for late-type dwarfs warrants additional study. Future research could aim to more precisely determine coefficients similar to those in Table 16, from which an estimate of the age of a FGK dwarf could be derived using (FUV − B) and B − V color indices. Further work is needed to determine whether such coefficients might depend on stellar metallicity. The (FUV − B, B − V) two-color relation does differ significantly between Population I dwarfs and metal-poor Population II subdwarfs as shown by Smith (2018b). However, additional work is needed to determine whether the coefficients might be metal dependent within the narrower metallicity ranges among thick disk and thin disk dwarfs.

Whereas GALEX FUV magnitude measurements have been combined with Johnson BV photometry to produce a FUV-activity index for the purposes of this paper, analogous formulations could be made by instead combining the FUV data with magnitudes and colors from other photometric systems. For example, in the case of FGK dwarfs the b and y magnitudes from the Strömgren system could be used to generate an (FUV − b, b − y) two-color diagram. The Gaia DR2 photometric system has passbands (Evans et al. 2018), that provide both a green G and a blue G_BP magnitude, which could potentially be used to develop a (FUV − G_BP, G_BP − G) two-color diagram for tracing stellar activity among late-type stars. Indeed a cross-correlation of GALEX and Gaia photometry could provide stellar activity information for vast numbers of stars. Our current approach in combining FUV with B and V magnitudes is in keeping with some other investigations of the stellar-activity sensitivity of the GALEX FUV passband (e.g., Smith & Redenbaugh 2010; Findeisen et al. 2011; Smith et al. 2017).

The main results of this paper have been derived by analysis of ensembles of stars. In assessing group membership probabilities for individual stars a combination of several different criteria are often applied, for example, King et al. (2003). This paper has not attempted to address the question of whether the addition of FUV photometry to some combination of other data, such as parallaxes, space velocities, CMDs, and spectroscopic metallicities, could tighten the determination of group membership probabilities for individual stars. Such a question would require quite different types of statistical analyses to the methodology followed here. As noted in Section 1, FUV photometry has been applied to the identification of late-type stellar members, particularly of spectral type M, in the case of relatively young moving groups (Rodriguez et al. 2013). Given the age-sensitivity of the GALEX FUV magnitude that has been demonstrated in this paper, there may be practical consequence to adding FUV photometry to membership determinations of late-F, G, and K dwarfs, even in kinematic groups much older than those studied by Rodriguez et al. (2013).

G.H.S. gratefully acknowledges the support of the National Science Foundation through award AST-1517791. This research has made use of the NASA Astrophysics Data System. Some of the data presented in this paper were obtained from the Mikulski Archive for Space Telescopes (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This work has made use of data from the European Space Agency (ESA) mission Gaia,⁵ as processed by the Gaia Data Processing and Analysis Consortium (DPAC⁶ ). Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France, as well as the General Catalog of Photometric Data operated at the University of Lausanne, Switzerland.