a catalog of galex ultraviolet emission from asymptotic...

A Catalog of GALEX Ultraviolet Emission from Asymptotic Giant Branch Stars

Rodolfo Montez, Jr.1, Sofia Ramstedt2, Joel H. Kastner3, Wouter Vlemmings4, and Enmanuel Sanchez51 Smithsonian Astrophysical Observatory, Cambridge, MA 02138, USA

2 Department of Physics and Astronomy, Uppsala University, Box 516, SE-75120, Uppsala, Sweden3 Chester F. Carlson Center for Imaging Science, School of Physics & Astronomy, and Laboratory for Multiwavelength Astrophysics, Rochester Institute of

Technology, 54 Lomb Memorial Drive, Rochester, NY 14623, USA4 Department of Space, Earth and Environment, Chalmers University of Technology, Onsala Space Observatory, SE-439 92, Onsala, Sweden

5 Department of Physics, University of Cincinnati, 400 Geology/Physics Building, Cincinnati, OH 45221, USAReceived 2017 January 13; revised 2017 April 21; accepted 2017 April 22; published 2017 May 22

Abstract

We have performed a comprehensive study of the UV emission detected from asymptotic giant branch (AGB) starsby the Galaxy Evolution Explorer (GALEX). Of the 468 AGB stars in our sample, 316 were observed by GALEX.In the near-UV (NUV) bandpass (l ~ Å2310eff ), 179 AGB stars were detected and 137 were not detected. Only38 AGB stars were detected in the far-UV (FUV) bandpass (l ~ Å1528eff ). We find that NUV emission iscorrelated with optical to near-infrared emission, leading to higher detection fractions among the brightest, andhence closest, AGB stars. Comparing the AGB time-variable visible phased light curves to corresponding GALEXNUV phased light curves, we find evidence that for some AGB stars the NUV emission varies in phase with thevisible light curves. We also find evidence that the NUV emission and possibly the FUV emission areanticorrelated with the circumstellar envelope density. These results suggest that the origin of the GALEX-detectedUV emission is an inherent characteristic of the AGB stars that can most likely be traced to a combination ofphotospheric and chromospheric emission. In most cases, UV detections of AGB stars are not likely to beindicative of the presence of binary companions.

Key words: binaries: general – stars: AGB and post-AGB – stars: chromospheres – ultraviolet: stars

Supporting material: machine-readable tables

1. Introduction

Stars between 0.8 and 8Me, including our Sun, will gothrough the asymptotic giant branch (AGB) phase of stellarevolution. During the AGB phase, these large ( R R100 )and luminous ( L103 ) stars experience nuclear burning inshells and lose copious amounts of mass at rates reaching up to

- -M10 yr4 1. As a result of their cool photospheric tempera-

tures (

Ramstedt et al. (2012) to search for X-ray detections associatedwith AGB stars. As in Ramstedt et al. (2012), there are a totalof 468 unique AGB stars in our sample: 286 M-type Mirasfrom Little-Marenin & Little (1990); 171 AGB stars from thesamples of Schöier & Olofsson (2001), González Delgado et al.(2003), and Ramstedt et al. (2006); plus 11 mixed-type starsfrom the sample of Sahai et al. (2008). The C-type (C/O>1)star sample is nearly complete out to 500 pc. The S-type(C/O∼1) star sample is nearly complete out to 600 pc. Thecompleteness of the M-type (C/O

we consider any GALEX source within 3″ of an AGB star( q 3off ) as a GALEX detection of an AGB star. In Table 1we provide a catalog of all GALEX measurements of AGBstars. Columns (1)–(6) contain basic information on the AGB

star compiled from SIMBAD, including its name, coordinates,chemical type (“Type”), and V- and J-band magnitudes.Columns (7)–(14) present information compiled from theGALEX catalog, including an extinction estimate, survey mode,

Table 1Catalog of AGB Stars Associated with GALEX Sources

Object R.A. Decl. Type V J -( )E B V Survey Date tNUV NUV tFUV FUV qoffset(deg) (deg) (mag) (mag) (mag) (s) (mag) (s) (mag) (arcsec)

BC And 345.22121 46.51042 M 9.11 2.47 0.29 AIS 2006 Nov 02 132 19.23±0.08 182 >22.6 0.45BU And 350.91625 39.72692 M 10.50 2.05 0.13 AIS 2006 Aug 07 167 22.71±0.47 192 >22.6 2.15

MIS 2011 Nov 06 1297 23.80±0.45 — — 1.54R And 6.00812 38.57704 S 7.39 1.17 0.09 AIS 2006 Nov 06 83 20.30±0.16 107 >22.1 0.31SV And 1.08363 40.10995 M 7.70 3.57 0.10 AIS 2004 Aug 19 445 19.80±0.05 178 >22.5 0.85TU And 8.09471 26.02943 M 10.22 3.42 0.04 AIS 2003 Oct 07 90 21.64±0.33 92 >21.9 0.53

GII 2004 Oct 02 1211 20.50±0.06 1610 >23.2 0.83UX And 38.37001 45.65438 M 8.69 1.31 0.11 AIS 2006 Dec 14 149 20.16±0.11 87 >21.8 0.76W And 34.38734 44.30494 S 6.70 1.59 0.09 AIS 2006 Dec 14 78 20.96±0.44 41 >20.2 1.12

AIS 2006 Dec 14 84 21.69±0.37 104 >22.1 1.36MIS 2011 Nov 23 1280 21.59±0.14 — — 1.57

(This table is available in its entirety in machine-readable form.)

Table 2Limiting Magnitudes for Undetected AGB Stars with GALEX Observations

Object R.A. Decl. Type V J -( )E B V tNUV NUV tFUV FUV(deg) (deg) (mag) (mag) (mag) (s) (mag) (s) (mag)

IRC +10011 16.60827 12.59807 M — 7.44 0.03 95 >21.9 119 >22.2IRC +10216 146.98919 13.27877 C 10.96 7.28 0.05 6367 >23.2 8422 >23.2NSV 24833 294.75308 −16.86569 S — 4.34 0.16 132 >22.3 195 >22.6LEE 338 326.11993 73.63468 C 9.82 3.44 0.58 380 >22.9 60 >21.2C* 59 18.43567 62.96006 C 9.00 4.74 1.85 74 >21.5 — —AH And 31.47750 40.72408 M 9.30 4.16 0.07 70 >21.5 — —EY And 356.25967 43.92394 M — 3.55 0.10 90 >21.9 — —KU And 1.72058 43.08333 M — 3.04 0.09 72 >21.5 — —RS And 358.84059 48.63826 M 8.38 1.54 0.15 128 >22.3 — —RY And 350.15663 39.62056 M 10.00 3.48 0.13 1366 >23.2 198 >22.6

(This table is available in machine-readable form.)

Figure 2. NUV (left) and FUV (right) observed (reddened) magnitudes as a function of exposure time for all measurements in our sample. In each panel the dashedline indicates the limiting magnitude estimate described by the formula = - +-( )m t t125 23.25limit exp exp1 mag, and the three chemical subtypes are distinguished asfollows: M-type stars as circles, S-type stars as squares, and C-type stars as diamonds.

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The Astrophysical Journal, 841:33 (14pp), 2017 May 20 Montez et al.

date of observation, the NUV and FUV exposure times andobserved magnitudes, and the angular offset between the AGBstar position and the GALEX source position. The lack of anNUV or FUV exposure and magnitude is indicated by a dash(—) in the respective columns.

To establish AGB nondetections, we based the observationstatus of a given star on the presence of nearby field sources.We considered AGB stars with no field sources within ∼3′ asunobserved. For an AGB star to be considered as observed butnot detected, or undetected, we required that at least one fieldsource lie in the direction of the four major quadrants (NE, SE,NW, SW). AGB stars that did not have field sources in all fourquadrants were considered uncertain observations and manu-ally inspected. In all eight cases of the uncertain observations,we determined that the position of the AGB star was off theedge of the detector and thus not observed. Table 2 lists all ofthe undetected AGB stars observed by GALEX. As in Table 1,

columns (1)–(6) contain basic information on the AGB starcompiled from SIMBAD, including its name, coordinates,chemical type (“Type”), and V- and J-band magnitudes.Columns (7)–(11) present information compiled from theGALEX catalog, including an extinction estimate, the NUV andFUV exposure times, and observed limiting magnitudes(determined as described in the following paragraphs).Given the different depths achieved by the various surveys

performed by GALEX, we studied the potential impact ofexposure time on detected and undetected sources. We plottedthe observed magnitudes and effective exposure times of allNUV and FUV measurements of our sample (see Figure 2).These plots show that exposure time varies widely across thesample and suggest that the detectability of the faintest sourcesis a function of exposure time. A majority of the uniquedetections of AGB stars were made in the shorter exposuresthat were part of the AIS. However, we note that eight NUV

Figure 3. Distribution of detections (dashed line and green-filled region) and nondetections (dotted line and gray-filled region). In the four panels we display thecumulative distributions of AGB star magnitudes in various bandpasses. The 2MASS H and K bandpasses (not shown) are similar to the J bandpass.

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detections of AGB stars and three FUV detections of AGBstars are made possible by deeper exposures (>300 s); the restof the deep exposure detections have accompanying detectionsin the shorter AIS survey exposures. Overall, we find that thedetections and nondetections of our catalog of AGB stars areunbiased by the various survey depths.

We used the pattern in Figure 2 to estimate the limitingmagnitude as a function of exposure time. In particular, we findthat a suitable estimate of the limiting magnitude for both theNUV and FUV is described by the simple function

= - +-( )m t t125 23.25limit exp exp1 mag. For each nondetection,we used the maximum exposure depth in the NUV and FUVbandpasses in the limiting magnitude function to determine theobserved NUV and FUV limiting magnitudes (see Table 2).Using the same procedure, we estimated limiting magnitudes ofnondetections in the FUV bandpass for the stars in Table 1.

Overall, we find that 316 of the 468 AGB stars in our samplewere observed by GALEX. Of the 316 observed AGB stars,179 are detected in the GALEX NUV bandpass and 38 werealso detected in the FUV bandpass, while 137 were notdetected in either bandpass. All 179 AGB stars detected in theGALEX imaging observations, including detections in multipleobservations, are listed in Table 1, and nondetections are listedin Table 2. In our cross-correlation, we also found 10 AGBstars with GALEX grism spectroscopic observations, which wediscuss later.

4. Characteristics of the GALEX-AGB Sample

4.1. Detections and Nondetections

The characteristics of detections and nondetections amongthe GALEX-AGB sample can provide insight into the nature ofGALEX-detected UV emission from AGB stars. In Figure 3 wecompare the optical and NIR photometric properties of detectedand undetected AGB stars. In each panel we display thecumulative distribution of apparent brightness for severaloptical and infrared photometric bands of the detected and

undetected AGB stars.8 The distributions in Figure 3 suggestthat GALEX-detected AGB stars are approximately 2 magbrighter, on average, than those that are not detected. Given therelatively narrow range of temperatures and bolometricluminosities of AGB stars (Vassiliadis & Wood 1993), theapparent magnitudes of our AGB stars are a first-orderindication of their relative distances. Hence, the patterns seenin Figure 3 further suggest that the AGB stars undetected in theUV by GALEX are more distant, on average, than UV-detectedstars. This notion is further supported by the fact that ∼44% ofthe detected AGB stars have Hipparcos parallax measurements(van Leeuwen 2007) with signal-to-noise ratios >1.5, whileonly ∼5% of the undetected AGB stars have similarlysignificant parallax measurements. For the observed J-bandapparent magnitudes, which are the least affected by interstellarmedium (ISM) extinction of the bands considered, there is clearevidence that the fluxes are proportional to the distance to theAGB stars (Figure 4) and the brighter stars are more easilydetected in the UV. However, ISM extinction increases towardshorter wavelengths and will influence the overall UVdetectability.Next, we consider the distribution of the AGB stars

(detections and nondetections) in galactic coordinates. InFigure 5, we present the distributions in galactic latitude, b,and longitude, l. In galactic latitude, the AGB stars appear to benormally distributed with ~ b̄ 0 10 and FWHM∼50°.The nondetections follow a similar distribution but with anarrower FWHM (∼25°) compared to the entire sample. Incontrast, Figure 5 shows that galactic latitudes of the detectedAGB stars display a bimodal distribution with a dip in thenumber of detected stars near b∼0. Although GALEX initiallyavoided low galactic latitudes, more of the galactic plane wasobserved toward the end of the extended mission (Bianchi2014). Galactic scale heights based on the Hipparcos-derived

Figure 4. Left panel: distances and scale heights of detected (green symbols) and undetected (gray symbols) AGB stars with Hipparcos parallax measurements withS/N>1.5. The series of lines indicate galactic latitudes of b=0° (dashed), ±30° (dotted), and ±90° (solid). Right panel: J-band magnitudes as a function ofdistance for the same sample. Solid gray line indicates an assumed distance modulus with + =( )M A 6.25 magJ J and D + = ( )M A 1.0 magJ J indicated by thedashed lines. In both panels, the chemical subtypes are indicated as in Figure 2.

8 We only provide the J band of the three Two Micron All Sky Survey(2MASS) bandpasses (J, H, K) because they are all very similar in appearanceand correlation.

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distance of detected and undetected AGB stars (see Figure 4)suggest that the bimodal distribution of AGB star detectionswith galactic latitude is not due to the poor coverage of thegalactic plane. Specifically, we find that AGB stars with highgalactic latitudes, ∣ ∣b 45 , are more readily detected at largerdistances than those AGB stars with lower galactic latitudes,

∣ ∣b 45 . Such behavior suggests that high galactic latitudesare more favorable sightlines for detections. This, in turn,suggests that ISM extinction, which increases more rapidlywith distance for sightlines in the galactic plane, is responsiblefor the decline in AGB star detection fraction with galacticlatitude. Indeed, that such a trend is due to dust is consistentwith studies of larger unbiased samples of GALEX sources suchas Bianchi et al. (2011).

To mitigate the influence of ISM extinction in subsequentanalysis, we use the extinction estimates provided by theGALEX source catalog and listed in Table 1 to dereddenthe observed magnitudes. A caveat of such an approach is thatthe GALEX extinction estimates are based on galactic dustmaps (Schlegel et al. 1998) intended to give total Galacticextinction for a given line of sight through the Milky Way;hence, these extinction estimates are likely to overestimate theextinction for the closest AGB stars that are near the galacticplane. Based on the galactic distribution of detections andnondetections, such a reddening correction will be problematicfor a majority of undetected sources and

site of UV emission. To estimate the selective extinction in agiven bandpass, -A EBP B V, we followed the proceduredescribed in a GALEX study of hot stars (Bianchi 2011) forMilky-Way-type dust with RV=3.1. However, given thenature of our sample, we used cooler stellar atmosphere models(

dereddened NUV and V-band magnitudes is consistent withcorrelated fluxes observed noncontemporaneously. A moreprecise determination of the scaling relationship between NUVand V magnitudes requires contemporaneous multiwavelengthobserving campaigns.

4.3. Spectroscopy

Ten AGB stars in our sample were observed with the slitlessGALEX grism (see Table 4 for information on grismobservations). All objects with spectroscopic observationswere detected in the NUV grism bandpass except for IRC+10216; only Mira is detected in both NUV and FUV grismbandpasses. Collectively, the observed spectra are fairlyhomogeneous, displaying some continuum emission withemission lines such as Mg II (∼2800Å) and C II] (∼2325Å).For some sources the continuum may not be detected

throughout the NUV grism bandpass, leading to a “flat”appearance toward longer wavelengths. We note that theemission lines are multiplets but the grism spectroscopy hasinsufficient resolution to resolve the individual lines. In Figure 9we display each grism spectrum along with the location ofthese two common UV emission lines. Only two stars, R UMaand VY UMa, do not display the Mg II emission lines, while RUMa also displays a distinct broad feature in the vicinity of theMg II location.

5. Discussion

We now consider the possible origin(s) of the UV emission.There are two main categories for the possible origin(s) of theUV emission: extrinsic and intrinsic origins. Extrinsic originsmight include scattering of interstellar radiation field (ISRF) bythe circumstellar shell or processes related to hot main

Figure 6. Dereddened NUV fluxes vs. B-, V-, R-, and J-band photometry. In each panel, the gray lines represent a simple scaling relationship (slope of unity) withoffsets of −9±1 mag (B band), −11±3 mag (V band), −12±3 mag (R band), and −18±3 mag (J band). The black dashed lines are linear fits of eachdistribution. Magnitudes were dereddened using the extinction values in Table 1. In all panels, the chemical subtypes are indicated by distinct symbols as described inFigure 2.

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sequence or evolved companions to the AGB stars, such as hotphotospheres or accretion disks. Intrinsic origins include thephotospheric and chromospheric radiation from the AGB star.Although shocks in the circumstellar material are possible,

these shocks are not expected to reach UV-emitting tempera-tures, so we do not consider them. In the following sections wediscuss each of these possible origins given the observed UVdetection rates and characteristics of the GALEX-AGB sample.

Figure 7. Dereddened FUV fluxes vs. B-, V-, and J-band photometry. The four panels display the relationship between the FUV flux and other bandpasses.Magnitudes were dereddened using the extinction values in Table 1. The lines and symbols are the same as in Figure 2.

Table 4AGB Stars with GALEX Grism Spectroscopy

Object Obs. Date tNUV (s) tFUV (s) Detected

EP Aqr 2007 Feb 07 2759 2759 NUV onlyRW Boo 2006 Nov 01 5113 1721 NUV onlyAA Cam 2005 Jan 30 3224 1693 NUV onlyMira (omi Cet) 2006 Nov 18 11325 11325 NUV and FUVV Eri 2005 Nov 03 1704 1616 NUV onlyTW Hor 2006 May 07 1088 1007 NUV onlyV Hya 2005 Aug 17 2696 2696 NUV onlyIRC +10216 (CW Leo) 2008 Jan 23 8761 8760 NondetectionR UMa 2006 Jan 06 1703 1703 NUV onlyVY UMa 2006 Jan 07 1704 1704 NUV only

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5.1. Extrinsic Origins

5.1.1. Scattering of the Interstellar Radiation Field

Scattering of UV photons from the ISRF by the dustycircumstellar envelope of an AGB star is a potential extrinsicorigin for the UV photons detected from that star. It has beensuggested that the UV emission from the ISRF influences thechemistry within the circumstellar envelopes of AGB stars(Decin et al. 2008). The dust-rich environment means that anyscattering would preferentially be in the forward direction andsome fraction can be scattered into our direction. Weconsidered the “standard” UV ISRF flux (Draine 1978) andestimated the maximum ratio of scattering to bolometricluminosities for the sample of AGB stars with reliable distanceestimates(see Section 2.3). Based on simple assumptions (e.g.,

spherical envelope geometry, standard ISM grain scatteringefficiencies), we find that unphysically large scattering radii(thousands of R*) with 100% efficiency are required to accountfor the measured NUV fluxes. For the closest objects, suchlarge scattering radii would produce extended UV sources thatare not detected in the GALEX images. Also, if scattering of theISRF is a dominant process for UV emission from AGB stars,then the scattering conditions implied by the NUV suggest thatFUV fluxes should be brighter than observed by two orders ofmagnitude. Additionally, it is difficult to reconcile the UVspectral signatures of the grism observations (continuum withemission lines; Figure 9) with scattering of the ISRF. Weconclude that although scattering of the ISRF is expected to bepresent, it is unlikely to be a significant source of the UVemission from AGB stars.

Figure 8. Selection of phased AAVSO visual light curves (see footnote 6) of AGB stars with at least three GALEX observations and sufficient AAVSO measurementsto clearly display the variability. In each panel we have arbitrarily shifted the dereddened NUV magnitudes (green symbols) to best match the visual magnitudes (graysymbols). Symbols are the same as in Figure 2.

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5.1.2. Binary Companions

UV emission from AGB stars has been suggested as apotential tool to detect binary companions from AGB stars(Sahai et al. 2008; Ortiz & Guerrero 2016). Over most of theelectromagnetic spectrum luminous AGB stars will outshinemain-sequence and/or post-AGB companions. However,because the photospheric radiation of an AGB star is expectedto drop rapidly toward short wavelengths (

temperatures obtained from their three temperature estimationmethods (i.e., NUV excess, FUV excess, and the ratio of theseexcesses). These discrepancies led the authors to conclude thatsome of the putative companions might be influenced by theAGB star circumstellar material or that the UV emission isproduced by an alternative process.

5.2. Intrinsic Origins

By analyzing a nearly complete, volume-limited sample ofAGB stars and thereby avoiding the biases inherent in previousstudies that employed smaller samples, we find that thebrightest (closest and most well studied) AGB stars have higherdetection fractions in the NUV and FUV than more distantAGB stars (Section 4.1). This suggests that the GALEX-detected UV emission from AGB stars is most likely inherentto the AGB star and not due to extrinsic processes. Indeed, IUEobservations reveal the prevalence of chromospheric-like UVemission from cool giants and supergiants (Luttermoser 2000).The photospheric models adopted by Sahai et al. (2008) andOrtiz & Guerrero (2016) do not include radiation from suchoverlying chromospheric emission, and it would appear likelythat such a process is responsible for AGB UV emission inmany, if not most, cases.

The faint UV fluxes, apparent scaling relationships withother bandpasses, and phase correlations between visual andUV magnitudes suggest that the observed UV emission couldarise from the Wein tail of the photospheric spectral energydistribution. However, we note that the behavior of thephotosphere in these regimes is poorly understood and thegas and dust of the circumstellar envelope are strong sources ofUV absorption. Hence, although photospheric radiation maycontribute to the GALEX-detected UV emission, it is unlikelyto account for all of the observed UV flux.

NUV to FUV spectroscopy from IUE and HST of AGBstars reveals that emission lines from Mg II, C II], and Fe IIare prevalent. Such emission lines are indicative of a

chromospheric layer in the atmospheres of these stars thatpersists into these highly evolved stages, perhaps neverdropping below a basal level (Schrijver 1995). The chromo-spheres appear to be much cooler than solar-like chromo-spheres. The heating mechanism is uncertain, but could be tiedto pulsations, wave heating, or magnetic fields (see review bySchrijver 1995, and references therein). The apparent pre-valence of UV emission from AGB stars (Figure 3), theapparent correlation with stellar fluxes (Figure 6), and thespectral characteristics of past IUE and recent GALEX grismspectroscopic observations (Figure 9) suggest that chromo-spheres are significant contributors to the GALEX-detected UVemission from AGB stars.Since the photospheres and chromospheres reside within

the circumstellar envelope, the properties of the UV emissionshould be influenced by absorption by the circumstellarmaterial. To explore this notion, we compiled mass-lossrates, Ṁ , and gas kinematics, vexp , for the Hipparcos sampleof AGB stars (Groenewegen et al. 1999; Schöier &Olofsson 2001; Olofsson et al. 2002; González Delgadoet al. 2003; Ramstedt et al. 2009; Danilovich et al. 2015) tocompute the density proxy, Ṁ vexp . In Figure 10 we comparethe dereddened and distance-corrected NUV and FUVemission with this mass-loss proxy for density. Despite thescatter, this comparison provides tentative evidence that thecircumstellar density and the dereddened distance-correctedNUV and FUV fluxes are anticorrelated. Performing a similarcomparison for other broadband measurements, we find thatthe anticorrelation increases for shorter wavelengths. Suchbehavior is expected if the circumstellar material residesbetween us and the source of UV emission and attenuates theflux. The evidence for multiple scaling laws suggested by theNUV correlations with optical and NIR emission could thenbe explained by varying degrees of absorption in the time-varying circumstellar shell.

Figure 10. Mass-loss density proxy and GALEX distance-scaled and dereddened magnitudes for the Hipparcos sample of AGB stars. The ratio of Ṁ and vexp isproportional to the density of the circumstellar envelope fed by mass loss. The data indicate that the NUV emission and, to a lesser confidence, the FUV emission areanticorrelated with the circumstellar envelope density. The black dashed lines are linear fits of each distribution. Magnitudes were dereddened using the extinctionvalues in Table 1. In both panels, the chemical subtypes are indicated by distinct symbols as described in Figure 2.

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6. Conclusions

We have searched for GALEX detections of a large numberof AGB stars culled from various catalogs. Among our sampleof 468 AGB stars, we find that 316 were observed by GALEX,179 were detected in NUV, and only 38 were detected in bothFUV and NUV. Based on our analysis of the GALEX data forthis sample, we determined the following.

1. Comparing the detected and nondetected samples, wefind that the brightest AGB stars have high NUVdetection rates (>90%) and the NUV detection ratedecreases with decreasing brightness. This suggests thatthe NUV is an inherent property of all AGB stars and thatthe brightest and, hence, closest AGB stars are morereadily detectable by GALEX. Studying the distribution ofour sample in galactic coordinates, we find that thenondetections are concentrated toward low galacticlatitudes, while the detections exhibit a bimodal distribu-tion in galactic latitude. Although GALEX did notcompletely cover low galactic latitudes, the bimodalbehavior seen in the distribution of galactic latitudes islikely due to the increased intervening ISM extinctiontoward the galactic plane, making detection moredifficult.

2. We find that the dereddened NUV fluxes appearcorrelated with dereddened broadband multiwavelengthphotometry (BVRJHK ), while the few FUV-detectedstars show no such correlation. The scatter in thesecorrelations is consistent with that expected fromnoncontemporaneous observing of correlated time-vari-able behavior. This notion is further supported byanalysis of phased AAVSO visual light curvesand multiple GALEX observations that suggest thatthe UV emission is correlated with the long-termpulsations observed from these AGB stars. Overall,these correlations suggest that the emission in the NUVbandpass is inherent to the AGB star. Future contem-poraneous optical and UV photometric and spectro-scopic measurements with good phase coverage arenecessary to improve our understanding of the physicalmechanism(s) responsible for the UV emission fromAGB stars.

3. UV spectroscopic detections (IUE, HST, and GALEX) ofemission lines from AGB stars suggest that chromo-spheres are present and contribute to the GALEX-detectedUV emission from most AGB stars. The processesresponsible for heating these chromospheres are anunresolved question, but if we assume that the prevalenceof UV emission indicates the ubiquity of such chromo-spheres, then their roles in heating and ionizing thecircumstellar environment and potentially driving massloss remain unexplored avenues of future study.

4. The anticorrelation between the circumstellar density andNUV fluxes further supports the notion that the UVemission originates from below the circumstellar shell,consistent with the chromospheric and/or photosphericorigin of UV from AGB stars. The multiple scalingrelationships suggested by the correlations of stellarfluxes with NUV flux can be interpreted as due to variousdegrees of extinction in the time-varying circumstellarenvironment.

The objects in our GALEX-AGB star catalog representexcellent targets for follow-up with UV spectroscopy andcontemporaneous optical observations. Future simultaneousmultiwavelength observations are urged to verify and expandour understanding of this emerging window into the AGBevolutionary phase. Such observations can be used to confirmthe origin of the UV emission from AGB stars, to establish themechanism for chromospheric heating, and to determine thelevel of magnetic activity at these evolved stages of stellarevolution.

This research has made use of the SIMBAD database,VizieR catalog access tool, and VizieR photometry tool12

operated at CDS, Strasbourg, France. The original descriptionsof the SIMBAD database and VizieR service were published inWenger et al. (2000) and Ochsenbein et al. (2000), respec-tively. We acknowledge with thanks the variable starobservations from the AAVSO International Database con-tributed by observers worldwide and used in this research. Thisresearch made use of Astropy, a community-developed corePython package for Astronomy (Astropy Collaboration et al.2013). Some/all of the data presented in this paper wereobtained from the Mikulski Archive for Space Telescopes(MAST). STScI is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555. Support for MAST for non-HST data is provided by theNASA Office of Space Science via grant NNX13AC07G andby other grants and contracts. We are grateful to theanonymous referee for insightful comments and suggestionsthat improved the manuscript. W.V. acknowledges supportfrom ERC consolidator grant 614264.Facilities: GALEX, AAVSO.

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1. Introduction2. Data2.1. The Sample of AGB Stars2.2. GALEX Observations2.3. Additional Data

3. Building the GALEX-AGB Sample4. Characteristics of the GALEX-AGB Sample4.1. Detections and Nondetections4.2. Correlation of GALEX-detected UV Emission with Other Bandpasses4.3. Spectroscopy

5. Discussion5.1. Extrinsic Origins5.1.1. Scattering of the Interstellar Radiation Field5.1.2. Binary Companions

5.2. Intrinsic Origins

6. ConclusionsReferences

a catalog of galex ultraviolet emission from asymptotic...

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