Deep LMT/AzTEC millimeter observations of Epsilon Eridani and its surroundings
M. Chavez-Dagostino, E. Bertone, F. Cruz-Saenz de Miera, J. P. Marshall, G. W. Wilson, D. Sanchez-Argüelles, D. H. Hughes, G. Kennedy, O. Vega, V. De la Luz, W. R. F. Dent, C. Eiroa, A. I. Gomez-Ruiz, J. S. Greaves, S. Lizano, R. Lopez-Valdivia, E. Mamajek, A. Montaña, M. Olmedo, I. Rodriguez-Montoya, F. P. Schloerb, M. S. Yun, J. A. Zavala, M. Zeballos
aa r X i v : . [ a s t r o - ph . E P ] J un MNRAS , 1–10 (2016) Preprint 9 July 2018 Compiled using MNRAS L A TEX style file v3.0
Deep LMT/AzTEC millimeter observations of ǫ Eridaniand its surroundings
M. Chavez-Dagostino ⋆ E. Bertone, F. Cruz-Saenz de Miera, J. P. Marshall, , G. W. Wilson, D. S´anchez-Arg¨uelles, D. H. Hughes, G. Kennedy, O. Vega, V. De la Luz, W. R. F. Dent, C. Eiroa, , A. G´omez-Ruiz, J. S. Greaves, S. Lizano, R. L´opez-Valdivia, E. Mamajek, A. Monta˜na, M. Olmedo, I. Rodr´ıguez-Montoya, F. P. Schloerb, Min S. Yun, J. A. Zavala, M. Zeballos, Instituto Nacional de Astrof´ısica Optica y Electr´onica Luis Enrique Erro School of Physics, University of New South Wales, Sydney NSW 2052, Australia Australian Centre for Astrobiology, University of New South Wales, Sydney, NSW 2052, Australia Department of Astronomy, University of Massachusetts, Amherst, MA 01003, USA Institute of Astronomy, University of Cambridge, Cambridge CB3 0HA, UK SciESMEX, Instituto de Geof´ısica, Unidad Michoac´an, Universidad Nacional Aut´onoma de M´exico, Antigua Carretera a P´atzcuaro 8701,Morelia, Michoac´an, CP 58089, M´exico ALMA SCO, Alonso de C´ordova 3107, Vitacura, Casilla 763 0355, Santiago, Chile Departamento de F´ısica Te´orica, C-XI, Facultad de Ciencias, Universidad Aut´onoma de Madrid, Canto Blanco 28049, Madrid, Spain Astro-UAM, Unidad Asociada UAM - CSIC, Madrid, Spain School of Physics and Astronomy, Cardiff University, CF24 3AA, UK Instituto de Radio Astronom´ıa y Astrof´ısica, Universidad Nacional Aut´onoma de M´exico, Antigua Carretera a P´atzcuaro 8701,Morelia, Michoac´an, CP 58089, M´exico University of Rochester, Department of Physics and Astronomy, Rochester, NY, 14627-0171, USA
Accepted . Received; in original form
ABSTRACT ǫ Eridani is a nearby, young Sun-like star that hosts a ring of cool debris analogousto the solar system’s Edgeworth-Kuiper belt. Early observations at (sub-)mm wave-lengths gave tentative evidence of the presence of inhomogeneities in the ring, whichhave been ascribed to the effect of a putative low eccentricity planet, orbiting close tothe ring. The existence of these structures have been recently challenged by high reso-lution interferometric millimeter observations. Here we present the deepest single-dishimage of ǫ Eridani at millimeter wavelengths, obtained with the Large Millimeter Tele-scope Alfonso Serrano (LMT). The main goal of these LMT observations is to confirm(or refute) the presence of non-axisymmetric structure in the disk. The dusty ring isdetected for the first time along its full projected elliptical shape. The radial extent ofthe ring is not spatially resolved and shows no evidence, to within the uncertainties, ofdust density enhancements. Additional features of the 1.1 mm map are: (i) the pres-ence of significant flux in the gap between the ring and the star, probably providingthe first exo-solar evidence of Poynting-Robertson drag, (ii) an unambiguous detectionof emission at the stellar position with a flux significantly above that expected from ǫ Eridani’s photosphere, and (iii) the identification of numerous unresolved sourceswhich could correspond to background dusty star-forming galaxies.
Key words: circumstellar matter – (sub-)mm: stars.
The circumstellar debris disks detected around mature,main sequence stars are a visible remnant of planet for- ⋆ E-mail: [email protected] (MC) mation processes (Backman & Paresce 1993). Composed oficy and rocky bodies ranging from micron-sized grains tokilometre-sized planetesimals, the presence of a disk is typi-cally revealed through the detection of excess emission fromthe star at mid- and far-infrared wavelengths (Wyatt 2008;Matthews et al. 2014). c (cid:13) M. Chavez-Dagostino et al.
Recent surveys by the
Herschel
Space Observatory(Pilbratt et al. 2010) have identified cool disks, analogs tothe Edgeworth-Kuiper belt of our solar system, in around20 ±
2% of Sun-like stars (Eiroa et al. 2013). However, theirdetection rate depended on both the temperature of the hoststar and on the observing strategy of the space craft. For in-stance, Thureau et al. (2014) found an incidence of 30% inA-type stars. The combination of exoplanet and debris disksurveys has provided evidence that planets are more com-mon around stars that also host a debris disk (Bryden et al.2013), and revealed tentative correlations linking the pres-ence of dust and planets with the properties of the host star(Wyatt et al. 2012; Maldonado et al. 2012; Marshall et al.2014a; Moro-Mart´ın et al. 2015). The presence of a planetaround a host star can be revealed through its dynam-ical interaction with the debris disk which creates non-axisymmetric structures (clumps, warps, cavities, etc.) inthe disk. Such structures led to the discovery of a giantplanetary companion in the prototypical debris disk host β Pictoris (Lagrange et al. 2010). Thermal emission fromthe micron sized dust grains dominates the observed fluxof the disk at far-infrared wavelengths, and exhibits typi-cal temperatures of 30 to 80 K (Morales et al. 2011) andradial size scales of 10s to 100s of astronomical units (AU)(Pawellek et al. 2014, see also Marshall et al. 2014a). Trac-ing the largest millimeter sized and coolest grains in the disk,which do not drift as far from their parent planetesimal beltunder the action of radiation forces as the smaller micronsized grains (Krivov et al. 2008), is vital to accurately de-termine the location of the dust-producing belt of planetes-imals around the star (Krivov 2010). Such measurementsare only possible with (sub-)millimeter continuum imagingobservations (e.g. Williams & Andrews 2006; Nilsson et al.2010; Pani´c et al. 2013). Resolving the radial extent of thedisk is fundamental in the modeling process as it directlyconstrains the orbital radius of the dust responsible forthe observed emission, weakening inherent degeneracies be-tween grain size and radial distance in those models reliantsolely on the disk thermal emission derived from analysesof the spectral energy distribution (SED; Augereau et al.1999; Lebreton et al. 2012; Ertel et al. 2014; Marshall et al.2014b). ǫ Eridani (”Ran”, HR1084, HD22049, HIP16537) isa relatively young (age=0.8 Gyr, Di Folco et al. 2004;Mamajek & Hillenbrand 2008; 1.4 Gyr, Bonfanti et al.2015), nearby ( d = 3 .
22 pc) Sun-like (spectral class K2V)star. Its age and distance place it as the closest isolatedstar of this kind where we can study the early stages inthe evolution of a planetary system analogous to the so-lar system. The star is host to a bright, extended, almostface-on debris disk, which ranks amongst the finest exam-ples of these objects so far discovered (Greaves et al. 1998;Holland et al. 1998). Recent models (Backman et al. 2009;Reidemeister et al. 2011) suggest that the disk is comprisedof up to four distinct components: two warm inner belts, acold outer belt and an extended halo of small grains. Inthese models the dust in the warm components actuallyoriginates in the the cold belt and is transported to the in-ner regions through the Poynting-Robertson drag and stellarwinds. Radial velocity analyses suggest the existence of twogiant exoplanets in addition to the warm inner debris disk(Hatzes et al. 2000; Moran et al. 2004). These planets are inferred to be within a few AU of the star, however theirexistence still remains contentious due to the high level ofactivity of ǫ Eridani, making interpretation of the spectro-scopic measurements difficult (Zechmeister et al. 2013).The cold outer belt of ǫ Eridani has been ex-tensively studied from far-infrared (FIR) to (sub-)mm wavelengths from the ground (Greaves et al. 1998;Sch¨utz et al. 2004; Greaves et al. 2005; Backman et al.2009; Lestrade & Thilliez 2015; MacGregor et al. 2015)and from space (Gillett 1986; Backman et al. 2009;Greaves et al. 2014). These observations showed that the de-bris disk has a ring-like morphology and provided the first es-timates of the basic physical properties of the ring such as ra-dial extent, width and inclination. Early sub-millimeter ob-servations conducted with the SCUBA camera on the JamesClerk Maxwell Telescope (JCMT, Greaves et al. 1998, 2005)also suggested that the ring has a clumpy structure thathas been interpreted as evidence of dynamical interactionbetween an unseen planetary companion and the debrisbelt (Quillen & Thorndike 2002). Substructures in the ringwere also identified in
Herschel /PACS images at 160 µ mby Greaves et al. (2014) who, after considering limb bright-ening effects due to inclination, obtained a 10% flux resid-ual when comparing the flux in the southern portion of thering to that of the north. Additionally, recent deep (rms ∼ ǫ Eridani,1 ′′ /yr, have enabled studies of the outer belt’s structureand dynamics, identifying which clumps in the belt couldbe associated with the disk and which might correspond tobackground sources, and to look for positional changes ofthe belt structures over time. The disk orbital motion hasbeen estimated to be of the order of 1 ◦ /yr (Greaves et al.2005) or three times as large (Poulton et al. 2006). Thereare, however, contrasting results. Observations at 1.2 mmwith the bolometer array SIMBA on the SEST telescope (ata depth of 2.2 mJy/beam rms) did not confirm the presenceof substructure in the ring (Sch¨utz et al. 2004), in agreementwith the very recent interferometric map at 1.3 mm collectedwith the Submillimeter Array (SMA) by MacGregor et al.(2015). They found that a smooth ring model could explaintheir patchy high resolution image. There are other exam-ples in which the presence of dust density enhancementsbased on early observations at long wavelengths have beenquestioned by more recent high resolution imagery. Note-worthy is the case of the very prototypical object Vega forwhich SCUBA observations in the sub-mm revealed brightblobs (Holland et al. 1998), but whose detection was laterdisputed (e. g. Hughes et al. 2012).Motivated by the debated presence of structure alongthe ring around ǫ Eridani, its potential correlation with aninferred planet orbiting close to the inner edge of the ring,and the possibility of measuring the orbital motion of dustenhancements within the ring, we conducted deep contin-uum observations at 1.1 mm with the AzTEC instrument
MNRAS000
MNRAS000 , 1–10 (2016)
MT/AzTEC observations of ǫ Eridani on the Large Millimeter Telescope Alfonso Serrano (LMT).In Section 2, we describe the LMT observations and reduc-tion techniques. In Section 3, the global observational mor-phology of the ring is presented. Section 4 is devoted to thedetailed modeling of the ring structure. In Section 5, webriefly discuss the spectra of the components of the system.Section 6 provides additional comments on the flux detectedat the stellar position, and in Section 7 we briefly commenton the background sources towards the ǫ Eridani’s system.The concluding remarks are given in Section 8.
Observations with the 1.1 mm continuum camera AzTEC(Wilson et al. 2008) were conducted in November and De-cember 2014 as part of the Early Science Phase-3 of theLMT, while the 50-m diameter telescope was operating inits 32-m aperture configuration. The telescope is locatedon top of the extinct volcano Sierra Negra, in the state ofPuebla, Mexico, at an altitude of 4600 m above sea level.A total of 18.5 hours on source were devoted to the tar-get under weather conditions that ranged from excellent togood ( τ GHz = 0.03–0.11). The field was observed with theAzTEC small-map observing mode which covers an area ofabout 7.5 arcmin . The point spread function (PSF) of theinstrument in this configuration has a FWHM beam sizeof 8.5 ′′ , however, filtering in the reduction process resultsin an effective resolution of 10.9 ′′ . We made observationsof the quasar 0339-017 roughly every hour which bracketedour observations of the ǫ Eridani field. The measured quasarpointing offsets (typically < ′′ ) were then interpolated intime to remove any offset and drift in pointing from thescience observations.The raw data were reduced using the standard AzTECanalysis pipeline and analysis approach (Scott et al. 2008;Wilson et al. 2008) but without applying the final Wienerfilter that many AzTEC observations use for the optimaldetection of point sources. Instead, the final unfiltered imagewas smoothed with a Gaussian filter with FWHM=6.8 ′′ . Thedata were flux calibrated based on observations of the proto-planetary nebula CRL618 and the noise in the final image isestimated from jackknifed time streams created as describedin Scott et al. (2008).Figure 1 displays the final AzTEC 1.1 mm map whichhas an rms of 0.20 mJy/beam, about ten times deeper thanthe SIMBA/SEST observations and four times deeper thanthose of MAMBO/IRAM. The scale of the map is 1 ′′ /pixelwhich results in 93 arcsec /beam for the smoothed beam sizeof 10.9 ′′ . This superb depth reveals three main interestingfeatures in the map: a well defined complete ring detected forthe first time at millimeter wavelengths, a clearly detectedcentral peak, and numerous unresolved background sources,some of which were present in previous maps, in particularthose from SCUBA/JCMT and Herschel /PACS and SPIREinstruments (Poglitsch et al. 2010; Griffin et al. 2010). Be-low, we describe in more detail these characteristics. The ring is detected (at a significance ranging from 5.7 to10.4 σ ) at all position angles and displays an almost perfectelliptical shape oriented along a north-south direction. Fit-ting an elliptical ring to the image results in a major axisof 20.0 ′′ (or 64 AU) and the minor axis of 16.9 ′′ (54 AU),which implies an inclination for a circular ring of about 32 ◦ .This latter value is in agreement with the same 32 ◦ derivedfrom Herschel data at 160 µ m (Greaves et al. 2014) and also,within their uncertainties, with the ∼ ± ◦ inclinationestimated from the SMA map (MacGregor et al. 2015). Thecentroid of the ring matches the stellar position to withinthe expected pointing uncertainties of the LMT, thereforewe conclude that no offset is detected between the ring and ǫ Eridani.The fact that we detect the full ring, clearly separatedfrom a central peak, allows us to provide more observa-tional constraints on the basic properties needed for the diskmodeling, namely: the distance from the star (the ring ra-dius R ), the ring width (upper limit), and the dust proper-ties obtained through the integrated flux density. In unre-solved disks, whose dust characteristics can only be inferredthrough analysis of the SED, some of the above derived prop-erties are, as mentioned before, degenerate.In addition to the ring radius R and the inclinationmentioned above, the ring appears unresolved in the radialdirection (width < ′′ or ∼
35 AU) and has an integratedflux density, obtained through standard aperture photom-etry analysis, of 27.7 ± ∼
160 degrees fromthe north counterclockwise reaching about 3.5 mJy/beam,which is ∼
50% above the lowest flux levels of the ring inthe 0–40 deg and 230–360 deg segments. This flux differenceis certainly less notable than the factor of three reportedin Greaves et al. (2005, their Fig. 3) and the factor of fourin Lestrade & Thilliez (2015). This SE bright arc is neitherseen in the MAMBO map nor in SMA data. Conversely,the apparent clump in the SW in the SCUBA and MAMBOmaps is not seen in the LMT image. While there are discrep-ancies, it is important to note that the published SCUBAdata provided the most complete ring maps in the imagingdata collected prior to these LMT observations, and thatthe bright arc in the SE has been regarded as a real featureof the ring. Whilst a direct inspection of the positions of theflux maxima of this SE arc in the two SCUBA and the LMTimages appear to indicate motion of substructures, the twosources east of the ring in the LMT map, in particular thatlabeled S3 in Figure 1, could conceivably explain the appar-ent flux enhancement in the SCUBA map of Greaves et al.(1998) as its position agrees with that of the ring at the timewhen the SCUBA observations were carried out. Similarly,the suggested background sources as the origin of the fluxbrightening in the SW in the first SCUBA map could nowwell partially contribute to the LMT SE brightening. In Fig-
MNRAS , 1–10 (2016)
M. Chavez-Dagostino et al.
Figure 1. ǫ Eridani. The outer ring is fully detected at a significance of > σ . The central peakis detected at 7.5 σ and likely corresponds to the sum of three contributing agents: the stellar photosphere, the stellar upper atmosphereand an (or perhaps two) inner warm disk(s). As many as seven (S1-S7) background objects are detected in this map, of which, four haveS/N >
5. The source labeled S1 is the brightest with a 1.1 mm flux of 4.6 ± ′′ FWHM is given bythe white circle in the top-left. As a reference, we include the contour levels for S/N = -3.5, -2, 2, 3.5, 5.0, 6.5, 8.0. ure 3 we show the positions of the dust ring with respect tothe stationary background sources at different epochs corre-spondent to previous (sub-)mm observations.To further verify the presence (or lack) of substructuresalong the ring, we modeled the ǫ Eridani LMT image usingoptically thin debris disk models as presented below. ǫ ERIDANI MODELLING
We modeled the emission from ǫ Eridani using a parame-terized model of an optically thin debris disk. This modelhas been described in detail in Wyatt et al. (1999) andKennedy et al. (2012). Basically, a 3D distribution of dustsurface area is generated, which can be viewed from any di-rection to create synthetic images. The surface brightnessof the images is calculated by adding up the emission in in-dividual cells along the line of sight, where each cell emitslike a blackbody at some temperature that is proportional tothe distance to the central star. For the resolved disk compo-nents we used a temperature law of T res = 416 / √ R K, where R is the disk radius in AU. Though we model the data at asingle wavelength, this temperature was chosen to provide a reasonable extrapolation of the model to the photomet-ric data at other wavelengths. For debris disks this modelcan account for important effects such as brightening atdisk ansae and brightness asymmetry for non-axisymmetricdisks.For a given model viewed at some orientation, a highresolution disk image is first generated. The central stellaremission of 0.7 mJy (i.e. the expected photospheric flux,see next section) is then added and the model is convolvedwith the LMT beam. A χ goodness of fit metric is thencomputed within an 80 × ′′ area shown in Figure 4, butonly pixels (about 70%) where the emission from either themodel or the image is significant are used (for details seeWyatt et al. 2012). The variable background level meansthat simple least squares minimization does not necessar-ily yield satisfactory results, so in most cases some by-handintervention was needed to obtain a smooth and continuousbackground. That is, even though models with χ valueslower than those presented are possible, they remove whatcould be astrophysical background behind the disk, in par-ticular at the SE side, and produce negative residuals in theNW. While the χ is always several times higher than thenumber of degrees of freedom (because our model of the sky MNRAS , 1–10 (2016)
MT/AzTEC observations of ǫ Eridani f l u x / bea m ( m Jy ) N NE E SE S SW W NW NPeak flux of the ringFlux minimum between star and ring
Figure 2.
Azimuthal flux distribution showing the variations ofthe flux maxima along the ring (continuous line). On average, thering flux has a level of 2.4 mJy/beam, and the brightest arc of thering is in the SE as can be seen in the map of Figure 1. The fluxpeak in this bright arc is ∼ Figure 3.
Zoomed version of the 1.1 mm LMT/AzTEC contin-uum map of ǫ Eridani. The map is presented in gray scales thatcorrespond to the contour levels of Figure 1. The ellipses denotethe ring positions at different epochs with respect to backgroundpoint sources due to the proper motion of the system. The el-lipses with thin solid, dot-dashed, dotted, and dashed lines showthe location of the ǫ Eridani system at the epochs of the obser-vations with AzTEC (2014.9), MAMBO (2007.9), and SCUBA(2002.0, 1997.9), respectively. Note that the source S3 almost co-incides with the ring position at the first SCUBA observations ofGreaves et al. (1998). near ǫ Eri is incomplete), visually better fitting models dohave lower χ values. As is commonly the case with modelsof low spatial-resolution data, we do not explore all possi-ble parameter space so do not claim that our models areunique, but that they are reasonable interpretations of theLMT data.The outer ring at ∼
70 AU is clear in the LMT im-age, so the main goals of the modeling were i) to determinewhether the ring width was well constrained, ii) determinewhether additional emission above that expected from thestar is present interior to the outer ring, and iii) look for anyevidence of azimuthal structure.Additional issues for this modeling were the overall fluxcalibration and any flux (DC) offset, and nearby backgroundsources. We allowed for a small DC offset in modeling theimages, though it did not influence the results. Near the diskthree background sources were added, one corresponding tothe blob in the NW, and two to the E and SE sources la-belled S3 and S2 in Figure 1, respectively. As we note below,there is the possibility of additional emission that exists be-hind the SE portion of the disk, which could plausibly be anextension of the diffuse emission to the E of the system.
To address point i) above, we constructed models with arange of ring widths with constant surface density cen-tered at a distance of about 70 AU from the star. Thesemodels consider an inclination of 30 ◦ which, unlike pre-vious observational results at millimeter wavelengths (e.g.MacGregor et al. 2015), is strongly constrained. The posi-tion angle is 7 ◦ from north to west (i.e. slightly west ofnorth), so the geometry is consistent with previous results(Greaves et al. 2014; MacGregor et al. 2015). Models witharbitrarily narrow ring widths reproduced the data reason-ably well, with χ values of around 4.7 (where χ is thereduced χ ), and these models also have a good fit to thedust emission interior to the main ring. We also constructedmodels with wide rings, finding that a width of up to 30 AUwas acceptable, but for larger sizes the ring surface bright-ness becomes too low, i.e. for a 30 AU width χ = 5.Some example models are shown in Figure 4. The lower tworows of panels show the results for narrow (10 AU) andwide (30 AU) outer belts, both of which produce reasonableresults. Thus, we conclude that the LMT image does notplace a lower limit on the ring width and that it could be aswide as 30 AU, conclusions that compare well with those ofLestrade & Thilliez (2015) and MacGregor et al. (2015). To address the second issue, we constructed models with anadditional interior component. The inclusion of this extracomponent is motivated by two facts: 1) we expect a con-tribution of an excess above the photosphere from the staroriginating from the warm belt(s) and 2) the significant fluxin the gap between the star and the ring we found in theLMT map (green dot-dashed line in Figure 2).The width and brightness of this component was variedto ascertain whether its existence was required by the data,and if so, the extent of this component. The above models
MNRAS , 1–10 (2016)
M. Chavez-Dagostino et al. that include only the outer belt leave significant residuals inthe interior regions ( χ = 8 . χ = 5 . χ ≈ . In all cases where a reasonable fit to the data was obtainedthere was no evidence for significant non-axisymmetric emis-sion in the residuals. In most cases emission remained in theSE part of the ring, but, given the extended structure seento the E of the system, attributing this residual emission tothe ǫ Eridani system is not well justified.Complementarily, we carried out a similar analysis asthat of MacGregor et al. (2015), namely, we calculated theresidual emission by subtracting a best fit model of the ringto the integral of the observational flux calculated in sectorswith a central angle of 10 ◦ extending from 10 to 28 ′′ from thecenter. Note that the outer radius of the annulus is slightlysmaller than in MacGregor et al. (2015) because we wantedto avoid contamination from the faint NW source close tothe ring. The azimuthal distribution of the residual flux isdisplayed in Figure 6. Note that the only potential featuredetected in the NE quadrant of the SMA map is not presentin our map. To within the uncertainties, the distributionshown in this figure indicates that the ring has a smoothstructure, with perhaps the presence of two regions of lowflux at azimuthal angles of ∼
220 and ∼
320 degrees.The various components in the best fit models have a1.1 mm flux of ∼
25 mJy for the outer ring, in good agree-ment with the photometric result shown above, and a totalof 5.5 mJy for the interior regions. A flux of 0.7 mJy is at-tributed to the stellar photosphere. For the model where theinterior disk component is a narrow ring the flux is 3.4 mJy,and when it is extended the flux is 4.5 mJy. ǫ ERIDANISYSTEM
The infrared to millimeter spectrum of the ǫ Eridani sys-tem is depicted in Figure 7. Three separate componentsare shown in this figure. The solid brown line representsthe photosphere that was generated from an interpolationwithin the library of synthetic spectra of Castelli & Kurucz(2003). These synthetic fluxes are calculated up to 160 µ m,so an extrapolation of the Rayleigh-Jeans tail was neces-sary to account for the flux at longer wavelengths. We haveassumed the stellar parameters derived by Paletou et al.(2015): ( T eff /log g /[Fe/H])=(5034/4.51/+0.16). The dashedcurve indicates the location of the modified blackbody curvefor a temperature of 48 K, λ =150 µ m, and an emissivity in-dex β =0.4 (Greaves et al. 2014). Ancillary data (see labelsin the figure) are also plotted for illustrative purposes andno formal fit of the points was attempted. The data, in spiteof some dispersion, are well represented by the above modi-fied blackbody parameters. The dispersion can be partiallyattributable to the differing components that are includedin the integrated flux densities calculated from distinct datasets. The dotted curve displays the best fit provided by the Herschel , SMA and LMT data for the inner component.The latter data point (1.3 mJy) includes only the unre-solved central emission after subtracting the photospheric(and chromospheric, see next section) contribution consid-ering λ =150 µ m. The resulting best fit parameters of thewarm component are T = 113 K and β = 1 .
0. The blue lineshows the extension of the modified blackbody for the innercomponent using the parameters of Greaves et al. (2014).The solid thick black curve corresponds to the summed con-tributions of the photosphere, the inner flux peak, and outerbelt.With the available data, dust masses ( M dust ) can becalculated for the different components. As in Greaves et al.(1998), we consider two cases for the absorption coefficient; k µm = 1.7 and 0.4 cm gr − which at 1.1mm correspondto k . mm = (850 / β × k µm cm g − . At a distance of3.22 pc, T =48 K, and β = 0 . k . mm = 1.53 and 0.36cm g − ), M dust for the cold outer belt is 0.0035 and 0.015M ⊕ , which are slightly smaller, but compatible with thosereported by Greaves et al. (1998).For the inner component we conducted two calculations:a) we first considered the best fit parameters obtained abovefor the uresolved warm component and the transformed ab-sorption coefficients k . mm = 1.31 and 0.31 cm g − for β = 1 .
0. The resulting M dust are 7.6 × − and 0.0003 M ⊕ for the two coefficient values, respectively; b) we assumed aninner component that includes both the unresolved compo-nent and a narrow warm disk with a total flux of 3.4 mJy, inagreement with an emissivity index of β =0.4. In this casethe dust mass is M dust =0.00017 and 0.0007 M ⊕ , for the highand low values of k . mm , respectively. According to theseresults there is a similar amount of warm dust ( 0.0001 M ⊕ )very close to the star and in between the central unresolvedemission and the external cold belt which could be, as men-tioned before, evidence of material being tranported to theinner regions from the outer relatively massive debris ring. MNRAS , 1–10 (2016)
MT/AzTEC observations of ǫ Eridani Model
Residuals ± σ contours Model
Residuals ± σ contours Model
Residuals ± σ contours Model
Residuals ± σ contours Model
Residuals ± σ contours Figure 4.
Models of the LMT 1.1 mm image of ǫ Eridani. Each row is a different model, with the panels showing, from left to right,the data, the model at the resolution of the 32-m LMT, the high resolution model, and the residuals. The residuals include contours inunits of S/N from ± σ . The panels of the first and second rows show an outer ring centered at 68 AU and 10 AU wide, with the stellarphotosphere (0.7 mJy) as the only contributor in the inner region (first row), and an artificial point source of 2.7 mJy added at the stellarposition (second row). The third row considers an outer ring as in the above panels and includes an inner ring 10 AU wide at 23 AU.In the fourth row the inner component is wider, extending from 14 to 63 AU, but follows a surface density power law with index=-3.5in order to keep it concentrated. In the bottom row panels we considered a wider (30 AU) outer ring centered at 69 AU, with a narrow(10 AU) inner component at 18 AU.MNRAS , 1–10 (2016) M. Chavez-Dagostino et al. S u rf ace b r i gh t n e ss ( m J y / b ea m ) Figure 5.
Radial flux distribution of the ǫ Eridani system. Thedotted lines are the Gaussian profiles fitted to the central peakand the ring. They both match the PSF of 11 ′′ , hence not re-solved. The continuous line is the sum of these profiles. Coloreddots represent the full set of flux points in each azimuthal direc-tion, and the average observed fluxes are depicted with the ”+”symbols. Note that significant diffuse flux is present in the gap at10 ′′ radius. b r i gh t ne ss ( m Jy / a r cs e c ) N NE E SE S SW W NW N Figure 6.
Flux residual obtained by subtracting a well fittingmodel flux to that of the observed map. The residual indicatesthat, to within the uncertainties, the ring has a smooth morphol-ogy. A reduced χ of the data points results in 0.92, supportinglack of substructures. The unresolved emission from the central peak is detectedat 7.5 σ . The integrated flux in the center is 2.3 ± µ m with Herschel /PACS (Greaves et al.2014). As mentioned above, the expected photospheric con-tribution to the total flux at 1.1 mm is 0.7 mJy, implyinga potential warm dust contribution of ∼ ∼ Wavelength [µm]10 -4 -3 -2 -1 F l u x D e n s i t y [ J y ] Data Source
IRASMIPSPACSSPIRESCUBAAzTECMAMBOSMA Warm ComponentCold ComponentBoth ComponentsPhotosphere
Figure 7.
Spectrum of ǫ Eridani. Different lines styles repre-sent the different components of the system: the photosphere(solid red), the outer belt (dashed), the inner component (dot-ted), and the summed contributions (thick solid). The instru-mental origin of the ancillary data is indicated in the figure insetand the symbols stand for data that contain fluxes for the innercomponent (triangles), the outer belt (circles), and those of thewhole system (diamonds). These data have been partially com-piled by Greaves et al. (2014) from Backman et al. (2009) andGreaves et al. (2005). The outer belt spectrum has been con-structed with the parameters T =48 K, λ =150 µ m, and β = 0 . Herschel
PACS at 70 and 160 µ m, the SMA data point at 1.3 mm, andthe LMT/AzTEC flux at 1.1 mm. The best fitting parametersassumming λ =150 µ m are T =113 K and β = 1 .
0. The solid blueline shows the long wavelength extension of the gray body emis-sion for the inner component considering an emissivity index of β = 0 .
4. The two data points at long wavelengths (and the upperlimit of MAMBO at 1.2 mm) provide meaningful constraints onthe dust properties of the warm belt. vided by the LMT and the SMA results in a much steeper β index and suggests that λ should remain close to the Her-schel /PACS band at 160 µ m. In addition, it has been foundthat active stars present a non-monotonic temperature vari-ation towards the upper atmospheric layers, in a similar waythe Sun displays a temperature increase towards low opticaldepths after reaching a minimum temperature in the atmo-sphere at about log τ = −
4. This temperature gradient re-versal is commonly evidenced by an excess in the ultravioletregime but also as an atmospheric (sub-)mm-wave excess.In a recent study, Liseau et al. (2015) found that α Cen B(K1 V) displays an excess at bands 9, 7 and 3 (440 µ m,870 µ m and 3 mm, respectively) of the Atacama Large Mil-limeter/submillimeter Array (ALMA). Considering that α Cen B and ǫ Eridani have similar spectral types and as-suming that both stars have a similar degree of activity, assuggested by their far and mid-ultraviolet (1200–2500 ˚A)continuum flux surplus, it is not unreasonable to expect asimilar excess in ǫ Eridani. The comparison of observed andpredicted fluxes of α Cen B indicates that in ALMA bands7 and 3 the observed stellar fluxes are, respectively, 40 and220% higher than predicted for the photosphere. Based onobservations conducted with the Australia Telescope Com-
MNRAS000
MNRAS000 , 1–10 (2016)
MT/AzTEC observations of ǫ Eridani pact Array (ATCA) at 7 mm, MacGregor et al. (2015) re-ported a flux excess of about a factor of three, which agreeswith the trend of an increasing excess at longer wavelengths.These authors argue that no plausible inner disk scenariois able to explain the observed excess and ascribe it to athermal origin in the upper atmosphere of ǫ Eridani. An in-terpolation of the bracketing ALMA points to the 1.1 mmband results in an excess of ∼ ∼ ǫ Eridani. This atmosphericvalue actually leaves 1.3 mJy at 1.1 mm originating fromthe unresolved warm belt(s) and, therefore, its contributionwould be about half of the measured central emission. Com-plementing the LMT and SMA data with measurements ofthe central source at other (sub-)mm bands will be veryvaluable to better assess the appropriate contributions ofthe sources of millimeter emission. These contributions willallow us to better constrain the dust properties of the warmbelt and to understand the outer atmospheric structure of ǫ Eridani.
As mentioned before, another important feature of ourmap is that, thanks to the depth and enhanced resolutionachieved, we found numerous sources whose nature has beensuggested, but never been previously investigated. In ad-dition to the cold ring and the central peak, seven pointsources were detected with a significance > σ and fluxesthat range from 1.2 to 4.6 mJy (see Table 1). These sourcesdo not have near- or mid-IR counterparts on either 2MASSor WISE images, but some are also present in previous
Spitzer (Rieke et al. 2004, MIPS-70 µ m;), Herschel (PACSand SPIRE at 70, 160, 250, 350 and 500 µ m), and, particu-larly, in SCUBA and SCUBA-2 images at 450 and 850 µ m.For several of the sources (S2, S3, and S7) our map providesthe first detection. It has been suggested by Greaves et al.(1998, 2005) that these sources are probably members of thepopulation of distant and heavily obscured sub-millimetergalaxies (SMG; Smail et al. 1997; Hughes et al. 1998). Con-sidering the area of the map, we expect ∼
10 SMGs to bepresent in our map for a detection threshold of S . mm = 0.7 mJy (Scott et al. 2012, Shimizu, Yoshida & Okamoto2012), which is consistent with the number of point sourcesdetected in the AzTEC map. It also agrees with the numbercounts derived in the very recent study of Fujimoto et al.(2016) scaled to the 1.1 mm wavelength for a spectral in-dex of 3. The analysis of these background sources is be-yond the scope of this paper. We, nevertheless, would like toremark that photometric and molecular line studies are re-quired to reveal the (distant or nearby) nature of the sourcessurrounding the high Galactic latitude ( | b | = 48 ◦ ) target ǫ Eridani.
We present the deepest (0.2 mJy rms) single dish observa-tions at millimeter wavelengths of the prototypical debrisdisk target ǫ Eridani conducted with the AzTEC camera onthe LMT. Our 7.5 arcmin image reveals the stellar emis- Table 1.
Positions and flux densities of background sources.Source offset RA offset DEC Flux[ ′′ ] [ ′′ ] [mJy/beam]Central peak ... ... 2.3 ± ± ± ± ± ± ± ± ǫ Eridani coordinates were α =03:40:13.1 and δ =-09:24:38.6 which include a correction forproper motion of µ α =-975.2 mas /yr and µ δ =19.5 mas /yr. sion, the cool disk, and nearby (line-of-sight) environmentwith the following features: • The ring is detected for the first time at all positionangles. The ring has a measured radius of 20 ′′ or 64 AU,and an upper limit of the width of 30 AU derived frommodel fitting, which implies a relative ring width (∆R/R)of • The central peak, which includes a stellar contributionand one or perhaps two warm dust belts is also clearly de-tected. These LMT observations, along with the recent SMAdata and archival
Herschel fluxes at 70 and 160 µ m, wherethe resolution is also good enough to separate the outer ringand the central peak, indicate that the interior warm dustcontributes approximately 60% of this emission. The theo-retical analysis of both the central peak and the outer dustring shows evidence of significant emission in the gap. Thismay constitute the first evidence of the Pointing-Robertsondrag outside the solar system. • Numerous point sources are detected around the ǫ Eri-dani system. The sources most likely correspond to a pop-ulation of massive distant star-forming galaxies as has beensuggested in previous works. Further analyses are neededto verify the nature and the properties of their cool dustcomponent.Our 1.1 mm observations demonstrate the current ca-pabilities of the operational LMT in the study of nearbycircumstellar debris disks. We have traced the full extentof the nearest debris disk/Edgeworth-Kuiper belt analog tothe solar system for the first time. At the same time wehave identified a number of line-of-sight background sources,which could be members of the sub-mm bright, high redshiftpopulation of star-forming galaxies. Disentangling these ex-
MNRAS , 1–10 (2016) M. Chavez-Dagostino et al. tragalactic sources from excesses around disk-host stars iscritical for the proper interpretation of planetary systemsanalogous to our own. The LMT is expected to operate atits full aperture capacity of 50 meters in 2017. The resolu-tion to be achieved in this final 50-m diameter configurationof the LMT is 5 ′′ , and when combined with the increasedsensitivity, stronger constraints on the ring properties willbe observationally established. ACKNOWLEDGMENTS
This work would have not been possible without the long-term financial support from the Mexican Science and Tech-nology Funding Agency, CONACyT (Consejo Nacional deCiencia y Tecnolog´ıa) during the construction and oper-ational phase of the Large Millimeter Telescope AlfonsoSerrano, as well as support from the US National ScienceFoundation via the University Radio Observatory program,the Instituto Nacional de Astrof´ısica, Optica y Electr´onica(INAOE) and the University of Massachusetts, Amherst(UMass). MC, EB, FCSM, MO and RLV work was sup-ported by CONACyT research grants SEP-2009-134985 andSEP-2011-169554. GMK is supported by the Royal Soci-ety as a Royal Society University Research Fellow. CE ispartly supported by Spanish grant AYA2014-55840-P. JPMis supported by a UNSW Vice Chancellor’s Postdoctoral Fel-lowship. SL acknowledges support from CONACyT throughgrant 238631. We are grateful to all of the LMT personneland observers from Mexico and UMass who made possiblethis project.
REFERENCES
Augereau J. C., Lagrange A. M., Mouillet D., Papaloizou J. C. B.,Grorod P. A., 1999, A&A, 348, 557Backman D. E., Paresce F., 1993, in Levy E. H., Lunine J. I., eds,Protostars and Planets III. pp 1253–1304Backman D., et al., 2009, ApJ, 690, 1522Bonfanti A., Ortolani S., Piotto G., Nascimbeni V., 2015, A&A,575, A18Bryden G., et al., 2013, in American Astronomical Society Meet-ing Abstracts 000