arXiv:1201.5067v2 [astro-ph.SR] 28 Mar 2012 agrbih-imdmlclrcod,eg,Srbn Gus Serabyn, e.g., clouds, molecular bright-rimmed larger ihaglrrslto bevtosso that show near-infrared, observations appear formation; resolution Nebula, angular ongoing Eagle to high the connected in be discovered to first EGGs, The rpysi h ro euaCutrrnefo 40–350 from of range rates Cluster 0.7–1.5 mass-loss Nebula and and Orion AU Sizes the in Johnstone, 1998). in- 1999; Bally their Hollenbach & and & Hollenbach, 2008), Storzer Soderblom (e.g. Hu & & terpretation Wen, Robberto, O’Dell, Ricci, (e.g. especially Cluster 1993; proplyds, Nebula Orion about the ex- found in those An be can . literature low-mass evaporating tensive young, to around referring disks as de- accepted circumstellar best generally the is and (photo- probably are fined, “” is disk) term material, protoplanetary The evaporating molecular around otherwise. de- surrounding disks ill-defined which in or rather terms, gas stars, These of off.” the low-mass ionization/evaporation as driven the behind is their scribe left them gas than are around dense so slowly of gas and more “globules surroundings, photoevaporated as density lower (1996) being are al. et that Hester ex- by defined EGGs, plicitly or material, globules molecular 1987 gaseous nearby evaporating Garay of forming and blobs dense see, 1979 ionize PIGS; Vidal gressively & i.e. Laques globules, e.g., ionized nearby (partially around stars disks mas- low-mass circumstellar from for- evaporate radiation ab- can star ionizing stars published the sive the nearby been phenomenon; in has to Much this formation about stars. detrimental star high-mass nearby be with of sence can compared when stars mation, B and O rpittpstuigL using typeset Preprint 2018 23, October version Draft 3 4 1 2 ic h aa eiwppr h rgnlPG aelargely have PIGs original the paper, review Garay the Since ainlRdoAtooyOsraoy 03Lpzil Roa Lopezville 1003 Observatory, Astronomy Radio National e rplinLbrtr,M 8-0,Clfri Institu California 183-900, MS Laboratory, Propulsion Jet eateto hsc n srnm,UL,LsAgls A9 CA Angeles, Los UCLA, Astronomy, and Physics of Department h rsneo togrdainfilsfo massive from fields radiation strong of presence The r nepee ntrso ntblte tteitraebtenth headings: between Subject interface so the former. or at the instabilities source of of wind eastwar terms distant heads in a their No interpreted of from are OB2 material pressure Cyg photoevaporated C ram the and by the ablated of shaped in field (ii) originated radiation and that West, ultraviolet cores the head-tail molecular by E-W ad dense photoevaporated common of a small, presence share are the that structures 20324+4057 show IRAS images, of vicinity IRAC the Spitzer archival and imaging, apl speoiatymlclr ihattlgsms xedn 3 exceeding ob mass line gas molecular total Our a with head. molecular, tadpole predominantly the is inside tadpole- Tadpole located limb-brightened, extended, nebula an near-infrar cometary show ground-based data and These HST, vations. with imaging optical resolution erpr ut-aeeghosrain ftefrifae sou far-infrared the of observations multi-wavelength report We HCE N CRHD H ALO APL NA INTERSTELLA AN IN TADPOLE A OF TAIL THE SCORCHED: AND SHOCKED × 10 A 1. T − E 6 tl mltajv 11/12/01 v. emulateapj style X introduction M ⊙ n uflw,IM niiulojcs RS20324+4057 IRAS objects: individual ISM: outflows, and tr:frain tr:pemi eune tr:protoplan Stars: sequence, pre-main Stars: formation, Stars: yr − 1 Hne ’el1999). O’Dell & (Henney .Sahai R. 4 rcnpro- can or ) rf eso coe 3 2018 23, October version Draft 1 raghvendra.sahai@jpl..gov .R Morris R. M. , ∼ e,&Mny1993 Mundy & ten, 5 of 15% eo ehooy aaea A91109 CA Pasadena, Technology, of te ,Scro M87801 NM Socorro, d, enrcs spold,adtearnmPGhsbe sdt d to used been has PIG acronym the and proplyds, as recast been ABSTRACT 0095-1547 1 ttetp fteeehn rns(cagra An- & of (McCaughrean advent trunks the elephant With the 2002). of dersen objects tips stellar young the there massive that at relatively evi- for and show evidence stars, also Eagle low-mass is the young of associated trunks” for “elephant dence the in EGGs the eetdfo h RSPitSuc aao sn color- a using ( Catalog (PPNs) Source criterion Point nebulae IRAS preplanetary the from long-lived selected candidate of relationship list its and EGGs. object or this PIGs, of proplyds, order to nature in 12-m the and elucidate SMT to millimeter- ARO imaging, the (sub) HST using and spectroscopy imaging, presents wave continuum paper imag- radio MIPS VLA This and ing, IRAC Spitzer cluster. spectroscopy, 5-m 8 Cygnus Palomar the the near No. with 20324) (I OB2 20324+4057 associated IRAS source nebula IRAS tadpole-shaped brightened, 2012). al. et et Koenig EGGs Smith 2010, and (e.g., al. “pillars”) regions star-forming as massive elephant- to many of towards referred presence (also structures wide-spread trunk the revealed have (WISE) veys Explorer Survey Infrared field ieetta hs on o Ps(..Shie al. et Sahai the (e.g. of PPNs fraction for quite found morphologies large reveal those a survey than that our IRASdifferent found in and resolved we HST the objects However, in revealed dust data. the to sources heated stellar required have local were that of presence objects band/8 ob- the (A All implying highly MSX counterparts, and is 2007). (JHK) dust 2MASS hot point-source al. have any et where (Sahai or scured source, distance some stellar at a located from dust cool relatively by dominated 2 ..Claussen M.J. , 02 a bevdi nHTiaigsre fa of survey imaging HST an in observed was 20324 I limb- extended, an discovered serendipitously have We s h ipe nteti fteTadpole the of tail the in ripples The ds. F hpdnbl ihabih,compact, bright, a with nebula shaped min idadtednemedium dense the and wind ambient e c RS23445,icuighigh- including 20324+4057, IRAS rce 60 d ilmtrwv n ai obser- radio and millimeter-wave ed, F / gu lu n r o en (i) being now are and cloud ygnus re oae oteWs,blowing West, the to located urces iinltdoesae bet in objects tadpole-shaped ditional retto:w rps htthese that propose we orientation: lse oae oteNorth- the to located cluster 8 . 25 3 .7 > evtosidct htthe that indicate servations M )coe oslc bet htare that objects select to chosen 1) ⊙ tr ik,IM jets ISM: disks, etary u ai continuum radio Our . 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2007). I20324 is one of the most prominent of these. It the NW lobe of the bipolar outflow. The NW outflow ex- was first detected as an emission-line nebula together with tends toward three faint red star-like objects (a, b and c in ′ ′ ′′ two other nebulous objects within a 3.1 × 3.1 field, in a Fig. 2): high-resolution (0.15 )HandKs band images ob- ground-based Hα + [NII] survey of candidate post-AGB tained with the Laser Guide Star Adaptive Optics system objects (Pereira & Miranda 2007: PM07). PM07 con- on the Palomar 200-in using the High-Angular Resolution cluded that all 3 objects are “related young stellar objects” Observer (PHARO) NIR camera show point sources at the and that the emission-line spectrum of I 20324 is indicative locations of a, b and c (Sahai et al. 2012, in preparation) of photoionization rather than shock-excitation. supporting the idea that these are stars, and not simply Independently, I20324 has been noted recently by compact disks illuminated by starlight from the Tadpole’s Wright et al. (2012: Wetal12), who included it among central star. The stars a and b show small “tail” struc- a group of 10 “proplyd-like objects” associated with the tures emanating from them (inset: Fig 2); the inner part Cygnus OB2 association. They considered both the pro- of star a’s tail is roughly aligned with the bipolar outflow plyd and EGG hypotheses, and concluded that neither axis, whereas in the outer parts, the tail starts curving scenario adequately explains their observations. They sug- backwards. gest that these and the other objects they identified are I 20324 falls within the region imaged by the Spitzer “a unique class of photoevaporating partially embedded Cygnus X Legacy Survey program (PI: J. Hora) with the young stellar objects”. In contrast, we find that our data IRAC (4 bands at 3.6, 4.5, 5.8, and 8.0 µm) and MIPS in- favor the EGG hypothesis. struments (2 bands at 24 and 70 µm) – since some of these data have been recently reported by Wetal12, we do not 2. observations & results discuss them in detail. Inspection of the Legacy Cygnus- 2.1. HST imaging X Spitzer images reveals a bright point-source in all four IRAC bands and the MIPS 24 µm band, together with a We obtained optical images of I20324 on 2006-07-22 us- tail structure. The coordinates of the point source in the ing HST’s Wide-Field Camera (WFC) of the Advanced Cygnus-X catalog are RA=20:34:13.23, Dec=41:08:14.1, Camera for Surveys (ACS), which has a plate scale of virtually identical with the location of the cometary knot 0′′. 050/pixel, using the F606W (λ = 0.60 µm, ∆λ = apex in the HST images. The 8 µm image is notable in 0.123 µm) and F814W (λ = 0.80 µm, ∆λ = 0.149 µm) showing the ripples on the South periphery of the Tad- filters. A 2-point dither was used for the imaging with pole (Fig. 1b), which are also seen in the HST image but a total exposure time of 2 × 347s per filter. These im- less clearly. Although the angular resolution is rather lim- ages show an extended nebula with a limb-brightened ited relative to the object’s size at 70 µm, the MIPS image “tadpole” shape (hereafter, we often use the label “Tad- shows a cometary shape with a bright head which is likely pole” to refer to I20324) (Fig.1a) oriented roughly E- dominated by flux from the central source in the Tadpole. W; the periphery of the nebula is distinctly brighter and We assume that I20324 is associated with the Cygnus OB2 more defined on the N side and shows prominent rip- association, and adopt a distance, D = 1.4kpc (Rygl et ples. A bright compact knot is located interior to the al. 2012)5. tadpole head (Fig. 2). The intensity of the knot, which it- self has a cometary shape, peaks at J2000 α=20:34:13.23, δ=41:08:14.6 (in the F814W image), coinciding with 2.2. Additional Multi-wavelength Observations the location of luminous point sources in the 2MASS Following the HST observations, we obtained support- (20341326+4108140: α=20:34:13.26, δ=41:08:14.07) and ing ground-based observations with (i) the Palomar 5-m MSX6c catalogs (G080.1909+00.5353: α=20:34:13.4, (ii) the Very Large Array (VLA) of the NRAO6, and (iii) δ=41:08:14). The axis of the cometary knot is aligned at a the 10-m (SMT, Mt. Graham, AZ) and 12-m (Kitt Peak, PA of −43◦(north through east). From the F606W image, AZ) (sub)millimeter-wave telescopes of the Arizona Radio we find that the Tadpole has a length of 54.7′′ (76,600 AU) Observatory (ARO). and a maximum width of 14.1′′ (19750 AU). The cometary The TripleSpec Spectrograph at Palomar was used to knot, seen more sensitively in the F814W image, is about observe I20324 with its 1′′ × 30′′ slit aligned N-S, so as to 2.4′′ × 1.3′′ (3350 AU×1800AU). The knot has a conical cut laterally across the Tadpole body, at two locations (i) shape near the apex with an “inner” opening angle of 55◦, passing through the cometary knot (Slit 1), and (ii) 12′′ E but then narrows towards a more cylindrical shape at off- of the knot (Slit2). The seeing was about 1′′ at K-band. ′′ sets greater than about 0.75 implying an “outer” opening Strong emission from H2 S(1), v=1–0 and other H2 lines angle of ∼ 0. was detected in both slits; for Slit 2, the emission is clearly The cometary knot has a diametrically-opposed faint limb-brightened (Fig. 1b). The Slit 1 spectra also show a counterpart seen only in the F814W filter – the simplest strong continuum at the position of the cometary knot, interpretation of these features together is that they repre- and extended emission in the HI Brγ and Paβ lines – the sent scattered light from the lobes of a collimated bipolar latter lines display a local peak centered on the knot. outflow directed along the polar axis of a flared disk (or The VLA was used in the C configuration to observe dense equatorial region) tilted such that the near-side of the field near the Tadpole nebula in August 2009, as part the disk lies to the NW of the bright apex of the knot. of program AS980. Observations were made at frequen- Extinction by the tilted disk then explains the faintness of cies 8.5 and 22.5 GHz (λ=3.6 and 1.3 cm, respectively). 5 Previous distance estimates to Cyg OB2 have been generally higher, with D ∼1.7 kpc (Hanson 2003); derived values of masses and luminosity in this paper scale as D2. 6 the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. 3

Standard data editing, calibration, imaging, and decon- free of significant emission. volution were performed using the Astronomical Imaging The spectra of the high-density tracer lines, HCO+ Processing System (AIPS) for both frequencies. For the J=3–2 and CS J=2–1 (Fig.4), show strong emission peak- −1 8.5GHz image, observed on 05August (Fig.3), the restor- ing at Vlsr = 10.4km s , indicating that the peak den- ing beam was 3.2′′ ×2.8′′ at P A = −65◦. The rms noise in sity in the emitting region is quite high7, ∼ 106 cm−3. the final image was ∼65 µJy beam−1. For the 22.5 GHz The map (5′ × 5′) of the CO and 13CO J=2-1 emission image (observed on 10 August), a 70 kλ taper was applied (with both lines being observed simultaneously) using the in the imaging step to provide a similar angular resolution “on-the-fly” mapping technique shows a compact source to the 8.5 GHz image. The VLA 8.5 GHz image shows a peaking at the same Vlsr (Fig. 5) centered on the Tad- tadpole-shaped structure at the location of I20324 and two pole (object A). A second nearby peak coincides with the additional sources, which were also detected in the PM07 Goldfish. The CO emission from I 20324 is extended along emission-line survey (objects B & C in their Fig. 1; object the E-W direction with a FWHM of about 45′′and it is A is I20324). IRAC 8 µm images (insets in Fig. 3) of ob- unresolved in the N-S direction; source B is unresolved. jects B & C show similarly elongated morphologies: given Sparse mapping of the HCO+ J=3–2 emission shows that its resemblance to a goldfish, we dub object B, the “Gold- it is also extended E-W. There is also extended, structured −1 fish”. We also searched for the water maser emission line emission in the velocity range Vlsr = −5 to 8km s that at 22.235 GHz towards the Tadpole, but did not detect any is not connected spatially or kinematically with the Tad- −1 emission over a total Vlsr range of ±41.85km s to a 1σ pole or Goldfish, but likely comes from the same complex sensitivity of 25 mJy beam−1 per 48.83 kHz wide channel. of progenitor molecular clouds that spawned them. The total flux densities at 8.5 GHz for the Tadpole, We derive an excitation temperature, Tex = 11K from Goldfish and object C are 55±1.2, 10±0.4, and 6±0.7 mJy. the peak intensity (TR = 6.2 K) of the CO J=2-1 line, At 22.5 GHz, only the Tadpole is detected with a flux den- assuming it to be optically thick. Assuming LTE con- sity of 30 ± 1.4 mJy. The significantly lower flux den- ditions, we find (using the RATRAN online code: Van sity at 22.5 GHz compared to that at 8.5 GHz for the der Tak et al. 2007), that the peak 13CO J=2-1 line in- −1 Tadpole implies the presence of non-thermal emission in tensity (TR = 2.5 K) and observed line-width (2 km s ), 22 −2 this source. The radio emission peaks strongly along the imply a total column density N(H2)=0.4 × 10 cm , 13 shock/ionization front at the head of the Tadpole, possibly assuming a standard interstellar CO/H2 abundance ra- as a result of a compressed magnetic layer in this front that tio of 10−6 (e.g., Hayashi et al. 1993). This gives a total is interacting with cosmic rays (CRs) associated with the molecular mass of 3.7 M⊙ for I 20324. Assuming a similar Cyg OB2 association – we note that the Fermi Large Area kinetic temperature and abundance for Source B, we find 13 Telescope has recently revealed a 50-parsec wide cocoon of its mass is about 1.3 M⊙ (its peak CO J=2-1 line inten- freshly accelerated CRs that fill the cavities carved in the sity is about half that of I20324). Our mass estimates are Cygnus star-forming region by stellar winds and ioniza- likely lower limits because the “standard” value adopted 13 tion fronts (Ackermann et al. 2011). Further observations for the CO/H2 abundance ratio may be too large since a at different radio wavelengths are needed to confirm and significant fraction of CO (and 13CO) may have been pho- explore the nature of non-thermal emission from the Tad- todissociated in a PDR that occupies much of the volume pole. of the tadpoles. We estimate a total mass of ionized gas in the Tadpole −2 of . 10 M⊙, assuming optically-thin free-free emission 3. the nature of i20324 at 22.5 GHz, and approximating the emitting region with The Tadpole is detected at all wavelengths from the a triangular slab (of area 375 arcsec2), which fits the Tad- near- to the far-infrared, with point-source counterparts pole’s projected shape in the 8.5 GHz image. Our mass es- in the 2MASS, MSX, IRAS, and Akari catalogs: the spec- timate is an upper limit since there may be a non-thermal tral energy distribution (SED)is shown in Fig. 6. We have contribution at 22.5 GHz. also included the IRAS Low Resolution Spectrum (LRS) The ARO data were taken during 2009 Jan-May. The spectrum for the Tadpole (with a correction of the abso- 12CO, 13COJ=2–1 lines and the HCO+ J=3–2 line were lute calibration applied as described by Cohen, Walker, observed with the 1 mm dual-polarization receiver employ- and Witteborn (1992)). Since the LRS data were rather ing ALMA Band 6 SBS mixers on the SMT 10-m, and fil- noisy, we increased the S/N ratio by binning the spectrum terbanks with 1MHz resolution. Typical system tempera- to a factor 3 lower resolution: a weak 10 µm silicate dust tures were 240K (770K) for the CO (HCO+) observations. feature can be seen in absorption. The IRAC photometry The beam size was θ = 32′′at 1.3 mm, and pointing ac- b was extracted from Cygnus-X catalog, whereas the MIPS curacy was estimated to be about ±4′′. The 12CO and photometry was derived from the Legacy Cygnus-X images CSJ=2–1 lines were observed with the 12-m telescope at using aperture photometry. The core of the 24 µm image Kitt Peak using dual–polarization SIS recievers. Typical of I20324 is partly saturated, so we used the bright Airy system temperatures were 430K (240K) for the CO (CS) ring at a radius of 26.5′′ from the center (4th ring) for observations, and data were recorded with the Millime- measuring the flux; our estimate of 37 Jy is consistent with ter Autocorrelator (MAC) configured to provide 48.8 kHz the IRAS flux value. Comparing the Spitzer photometry resolution. The beam size was θ = 60′′at 2.6 mm, and b with that from missions having lower angular resolution pointing accuracy was estimated to be about ±6′′. Obser- (red symbols) at similar wavelengths (24 and 70 µm), we vations were conducted in position-switching mode using find that the former (hereafter the “small aperture” fluxes) an off position 15′ South of I20324 which was tested to be fall systematically below the latter at λ ∼ 60 − 70 µm. 7 The critical density for collisional excitation of the HCO+ J=3–2 line is > 3.9 × 106 cm−3 in a cloud at a temperature of < 20 K 4

This indicates the presence of an extended source of warm (#9 in Wetal12), (iii) and an object at RA=20:34:45.9, dust at these wavelengths heated by the external radia- Dec=41:14:46.5 in the IRAC images of this region (#10 tion field, in addition to the compact central source, which in Wetal12). The common E-W orientation strongly sug- looks point-like out to a wavelength of at least 24 µm. The gests these structures are shaped by the ram pressure of large mid- and far-IR fluxes of this point-source imply the a passing wind from a distant source or sources located presence of a substantial mass of circumstellar dust around to the West of these objects, with material being ablated the Tadpole’s central star. The total source luminosity of from their heads and blown eastwards. Noting that the the Tadpole derived from integrating the “small aperture” tails in both the Tadpole and Goldfish are bright on their SED is about 500L⊙, implying that the Tadpole’s central northern sides (both in the optical and in the radio), we star is not a low-mass star. From an inspection of pre- conclude that this is due to asymmetric irradiation of these main-sequence stellar evolutionary tracks by Bernasconi objects by the ultraviolet radiation field of the Cyg OB2 & Maeder (1996), we find that our estimated luminosity No. 8 cluster that lies to NW of the Tadpole and Goldfish, ′′ implies a stellar mass of around 5M⊙ if the star has just at a separation of about 900 (∼6 pc). begun deuterium burning, or somewhat lower if it is ap- The Tadpole and its associates are most likely small, proaching the main-sequence. dense molecular cores near the Cyg OB2 complex that We fitted I20324’s “small aperture” SED using the on- are being subjected to by the strong line tool provided by Robitaille et al. (2007), which com- UV radiation field due to the large number of O stars putes least-squares fits of pre-computed models of young in this massive star formation region. The presence of stellar objects (YSOs) having disks and rotationally- additional young stars (objects a, b, and c: Sahai et flattened infall envelopes (with biconical outflow cavi- al. 2012, in preparation) in close physical proximity to ties), to user-defined SEDs. We set the input distance I20324’s cometary knot indicates that several young stars range to D=1.3–1.6 kpc. The input interstellar extinc- have formed within the dense molecular core inside the tion range was set to Av = 0 − 2, based on our esti- head of the Tadpole. The small tails associated with a mate of Av = 0.7 ± 0.7 using the numerical algorithm and b may represent (i) winds from these stars interacting provided by Hakkila et al. (1997), which computes the with the NW outflow of, or (ii) possibly photoevaporation 3-dimensional visual interstellar extinction and its error of their circumstellar disks by radiation from, the Tad- from inputs of Galactic longitude, latitude, and distance, pole’s central star. The curvature of a’s tail away from from a synthesis of several published studies. The best-fit the Tadpole’s head may be due to interaction with the model has D=1.4 kpc, Av = 1.5, and a central star mass, compressed “wall” of gas in the latter (note that the wall M∗ = 4.6 M⊙ and effective temperature, Teff = 4140 K; curves around both the back and the front, so a could be the model spectrum shows the presence of a weak silicate located quite close to the rear Tadpole wall). absorption feature as observed (Fig. 6). In this model, the The physical processes responsible for producing the ◦ full opening angle of the bipolar outflow cavity is θc =7 , radio-bright peripheries of the Tadpole and its associates and the outflow cavity axis is inclined at a modest an- are likely the same ones that operate in the vicinity of gle of i = 18◦, to the line-of-sight. The next best model the M16 (Eagle Nebula) elephant trunks (lateral width 18 has D=1.4 kpc, Av = 2, M∗ = 6.5 M⊙, Teff = 4200 K, ∼ (0.5−1)×10 cm) where dense, dusty molecular clouds ◦ ◦ θc =5 , and i = 18 , however the depth of the silicate fea- are being eroded by photoevaporation due to radiation ture relative to the continuum, is twice that observed. In from O stars at a distance of 6 × 1018 cm from the trunks, both models, almost all of the emitting mass is associated producing a photoevaporative flow. The heads of these with the envelope. These models are qualitatively consis- trunks appear in emission-line images as limb-brightened tent with the optical appearance of the luminous central arcs at the interface between the molecular cloud and the source, with scattered light from the inner regions of the ambient HII region. The bright heads (size ∼ 3 × 1017 cm) near-side bipolar cavity producing the cometary knot seen and bodies of the Tadpole and its associates likely repre- in the HST image, and its farside counterpart being ob- sent similar interfaces. scured by the dense inner equatorial region of the envelope. We interpret the ripples evident in the Tadpole body as The value of the de-projected opening angle in the best-fit resulting from Kelvin-Helmholtz (K-H) instabilities. Such −1 ◦ model, icorr = tan (tanθc/sini)=23 , lies in the range instabilities are expected to occur at the periphery of a covered by the inner and outer opening angles of the knot dense cloud embedded in diffuse gas, when there is rel- (0 − 55◦). We note that the minimum inclination angle ative motion between the two components (e.g., Fleck in the model is i = 18◦, so it is possible that a better fit 1984, Kamaya 1998). The spatial wavelength of the ripples might have been obtained with a smaller inclination angle (∼ 15′′ =3.2 × 1017 cm) is very similar to the wavelengths which would result in a larger value of icorr. However, a of the “ripples” found on the surfaces of some molecular better 10 µm spectrum covering the silicate feature (i.e., clouds in the by Bern´eet al. (2010) that have with higher S/N and a small aperture) is needed before also been interpreted as resulting from K-H instabilities. the derived parameters from the above modelling can be We consider (but discard) the possibility that the ripples put on a firm footing. could be caused by the orbital motion of two bound stars We note that there are several tadpole-shaped objects separated by a distance comparable to the ripple ampli- in the vicinity of I 20324 that share with the latter a com- tude (&2′′=2800 AU), since the ripple wavelength implies −1 mon E-W head-tail orientation. These are: (i) the Goldfish a time scale of 1050yr(100km s /Vw) (Vw is an assumed (outside the field-of-view in our HST images; #8 in We- speed for the exterior wind impacting the Tadpole), which tal12), (ii) “object C” of PM07 seen faintly in the 8.5 GHz, is much shorter than the expected orbital time for such a 5 −1/2 HST and Spitzer images, which has a relatively small tail binary system, 10 yr (M1 + M2) (taking the masses 5 of the stars in the binary to be M1 ∼ M2 ∼ 1 M⊙). Their relatively high internal density has not only pro- Although the bow-shock morphology of the head of the longed their survival in the hostile radiative and windy tadpole structure and the cometary shape of the central environment of the Cyg OB2 cluster, but it makes these knot in I20324 are very similar to the shapes of structures globules a likely place for stars to have begun forming, es- seen towards proplyds in the Orion star-forming region pecially as the overpressure of the surrounding HII region (e.g., O’Dell & Wong 1996, Bally et al. 1998), we do not would have aided gravity by compressing the globules. It think I20234 or object B are proplyds because the associ- therefore seems to be no accident that the Tadpole and the ated molecular masses (greater than 3.7 and 1.3 M⊙) are Goldfish have embedded stars. Wetal12 point out that a much too large compared to what one might expect for cir- higher fraction of the globules in this region (70 %) appear cumstellar material being evaporated from protoplanetary to have stars than those in the Eagle Nebula (15 %), and disks: disk masses of (0.003–0.07)M⊙ have been inferred they use this fact to argue that their objects in Cygnus are from an SMA continuum survey for the Orion proplyds therefore unlikely to be EGGs. However, several variables (Mann & Williams 2010). Indeed, no dense molecular can affect this fraction. The parent cloud of the Eagle medium has yet been found around the central disks in any Nebula may have been considerably less dense and mas- known proplyd. Furthermore, the minimum separation sive than the parent cloud of the globules in Cygnus, and between the bright periphery of the Tadpole head (which the Cygnus OB2 association could provide a much stronger would represent the wind shock in the proplyd hypothe- radiation field and more powerful winds than the cluster sis) and the cometary knot in I20324 is 4.7′′ = 6600 AU, is powering the Eagle Nebula. Both of these factors would much larger than the corresponding typical separations for affect both the size distribution of the resulting EGGs and the Orion proplyds (∼ 500 − 1000 AU: Bally et al. 1998). the compression to which they are subjected. However, this argument needs to be strengthened by a In summary, it is quite plausible that not only the Tad- quantitative assessment of the competing effects of (i) the pole and the Goldfish, but all of the “proplyd-like” objects larger number of ionizing stars in Cyg OB2 and, (ii) their listed in Wetal12 are EGGs, considering their rather simi- larger distance from the location of I 20324 and its as- lar physical sizes and shapes. Molecular-line observations sociates compared to Orion, since the physical scales of of these objects, like the ones reported in this paper, will structures associated with proplyds scale inversely with help in determining their masses and thus confirming their the ionizing EUV flux. true nature. We conclude that I20324 and its associates are EGGs. EGGs are likely to be the surviving dense portions of their 4. acknowledgments parent molecular clouds, and as such, they should be pre- We thank Anna Rosen for her valuable help in reduc- dominantly molecular, as observed in our objects. While ing the ARO on-the-fly data presented in this paper, as the original concept of EGGs shows them as having elon- part of her Spring 2009 NASA/USRP student internship gated tails that connect them continuously with the ele- at JPL. We thank the staff of the Arizona Radio Observa- phant trunks in M16, this is due to the ionizing sources tory for granting us observing time. The National Radio in M16 being concentrated in one direction relative to the Astronomy Observatory is a facility of the National Sci- EGGs. However, the presence of multiple ionizing sources ence Foundation operated under cooperative agreement by distributed over a large solid angle relative to the EGGs, as Associated Universities, Inc. RS’s contribution to the re- in Cyg OB2, results in a diffuse radiation field that eventu- search described here was carried out at the Jet Propul- ally pinches off their tails – a possibility noted by Hester et sion Laboratory, California Institute of Technology, under al. (1996), and supported by numerical simulations (e.g., a contract with NASA. Financial support was provided by Ercolano & Gritschneder 2011) – creating “free-floating” NASA through a Long Term Space Astrophysics award to EGGs, such as the Tadpole and its associates. RS and MM, and HST GO award 10536 to RS.

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(a) (c) N line flux

E

Slit 2 Slit 1 (b)

Fig. 1.— (a) Composite HST ACS/WFC (false-color) image (size 62.7′′ × 27.1′′) of I 20324 taken through two broad-band filters F606W (green) and F814W (red). (b) False-color IRAC 8 µm image of same field-of-view as in panel a: the intensity in a 19.2′′ × 18′′ box centered on the central star of I 20324 has been scaled by a factor 0.01 in order to clearly show the much fainter tail structure. Linear stretches have been used for the images in these panels. (c) Spatial intensity cuts of near-infrared emission lines as seen through a 1′′ wide slit, at two locations towards I 20324 – Slit 1: (green) 2.1 µm H2 v=1-0, S(1) & (red) 1.28 µm Pa β; Slit 2: (black)H2 v=1-0, S(1). The continuum emission from the central source has been subtracted in the Slit 1 cuts: the intensity of the H2 line in the region of the central source (which is very bright at 2 µm) has been masked out because the residuals from the subtraction process are much larger than the line intensity. 7

a c

b a

b

cometary knot

Fig. 2.— As in Fig. 1(a), but for a 8.5′′ × 8.5′′ region covering the cometary knot (a logarithmic stretch has been used to display each of the F606W and F814W images in the color composite). Inset shows the F814W image of the region around stars a and b in false-color to emphasize the faint tail structures. 8

N Object A (Tadpole) E

Object C

Object B (Goldfish)

Fig. 3.— VLA map of the radio emission at 8.5 GHz from I 20324. Panel size is 137.7′′ × 111.6′′. The beam is 3.2′′ × 2.8′′ and the rms noise is ∼65 µJy beam−1. The peak radio intensities towards the Tadpole, Goldfish and Object C are 1.33, 0.90, and 0.57 mJy beam−1. Insets show IRAC 8 µm images of objects B and C, on the same angular scale. 9

CO J=2−1 HCO+ J=3−2 (X 5)

CS J=2−1 (X 10)

TR (K) 13CO J=2−1 (X 2)

Vlsr (km/s)

Fig. 4.— Molecular line emission from I 20324, observed with the ARO’s 12-m and 10-m telescopes. For clarity, the 13CO J=2-1, CS J=2–1 and HCO+ J=3–2 lines have been rescaled; the rescaled CS and HCO+ lines have been shifted vertically by 1 K. The weak CO emission −1 centered at Vlsr ∼−2km s is due to emission from an extended molecular cloud (see Fig. 5). 10

A

B

Fig. 5.— Map of the CO J=2–1 emission towards I 20324, obtained with the ARO 10-m, with each panel showing the average emission over a 1.3km s−1 wide channel, centered at the LSR velocity shown in the top left corner. The small tickmarks on the x- and y-axes are spaced by 0.5′ and the contours are at 1, 2, 3, 4, 5, & 6 K. The CO emission clumps associated with the Tadpole (“A”) and the Goldfish (“B”) are labelled. 11

100

10

1 Flux (Jy) 0.1

0.01

10 100 Wavelength (micron) Fig. 6.— The spectral energy distribution of I 20324. The red circles show photometry from MSX, IRAS, and Akari , and the green circles show photometry from 2MASS and Spitzer – the systematic errors in the photometry (not shown) are smaller than the symbol sizes (which correspond to ±15% errors). Blue and magenta curves show the IRAS/LRS short-wavelength (SW: 7.7–13.4 µm) and long-wavelength (LW: 11–22.6 µm) spectrum. The black curve is the SED of a pre-computed disk-envelope model for a young stellar object with a central star of mass 4.6M⊙.