The Rosette Eye: the key transition phase in the birth of a massive star
aa r X i v : . [ a s t r o - ph ] J un The Rosette Eye: the key transition phase in the birth of amassive star
J. Z. Li
National Astronomical Observatories, Chinese Academy of Sciences, Beijing 100012,China; [email protected]
M. D. Smith
Centre for Astrophysics & Planetary Science, University of Kent, Canterbury CT2 7NH,UK
R. Gredel
Max-Planck Institut f¨ur Astronomie, K¨onigstuhl 17, D-69117 Heidelberg, Germany
C. J. Davis
Joint Astronomy Centre, 660 North A‘ohoku Place, Hilo, HI 96720
T. A. Rector
University of Alaska at Anchorage, 3211 Providence Drive, Anchorage, AK 99508
ABSTRACT
Massive protostars dramatically influence their surroundings via accretion-induced outflows and intense radiation fields. They evolve rapidly, the disk andinfalling envelope being evaporated and dissipated in ∼ years. Consequently,they are very rare and investigating this important phase of early stellar evolutionis extremely difficult. Here we present the discovery of a key transient phasein the emergence of a massive young star, in which ultraviolet radiation fromthe new-born giant has just punctured through its natal core. The massiveyoung stellar object AFGL 961 II is readily resolved in the near infrared. Itsmorphology closely resembles a cat’s eye and is here dubbed as the Rosette Eye.Emerging ionized flows blow out an hourglass shaped nebula, which, along withthe existence of strong near-infrared excess, suggests the existence of an accretiondisk in the perpendicular direction. The lobes of the hourglass, however, arecapped with arcs of static H emission produced by fluorescence. This study hasstrong implications for our understanding of how massive stars embark on theirformation. 2 – Subject headings: stars: formation – stars: early type – stars: individual (AFGL961 II) – accretion, accretion disks – ISM: jets and outflows
1. Introduction
Protostellar objects are, in most cases, deeply embedded in molecular clouds and areenshrouded by heavy foreground extinction. This makes the very early stages of stellargestation notoriously illusive (Zinnecker & Yorke 2007). The birth of stars with massesabove 10 M ⊙ is particularly intriguing as their radiation is apparently sufficient to resist theaccretion of gas and, hence, further mass growth unless somehow mitigated (Mckee & Tan2002; Behrend & Maeder 2001). Evidence of potential disks and/or envelopes associatedwith massive star formation has been accumulated through various probes especially in theradio domain (Chini et al. 2004; Patel et al. 2005). However, the formation process ofmassive stars remains far from being resolved. The critical growth period of massive starslasts only tens of thousand of years but is usually accompanied by spectacular ejections ofgas in opposite directions. Since such jets are circumstantial evidence of an accretion disk, itis possible and crucial to gain robust evidence of ongoing accretion associated with massiveyoung stellar objects (YSO) in their early stages of evolution.The Rosette Molecular Complex (RMC) is a famous isolated massive star forming regionwith an extent of about 100 pc. It is located at a distance of ∼ ∼ M ⊙ (Blitz & Thaddeus 1980). New generationOB star formation is in evidence in the densest ridge of the Complex (Li & Smith, 2005).The well known high-mass protostellar system AFGL 961 is situated well within this region(Cohen 1973; Grasdalen et al. 1983; Castelaz et al. 1985; Lenzen et al. 1984; Hodapp1994). The third component of the system, designated as AFGL 961 II (Li & Smith 2005),was first noticed to be associated with a small nebulosity by Eiroa (1981) and later brieflydiscussed by Hodapp (1994). The nebulosity was considered as a cavity with a partial shellsurrounding the central young star (Aspin 1998; Alvarez et al. 2004). However, the originof the intriguing YSO is far from clear.
2. Observations and Data Reduction2.1. Infrared imaging and spectroscopy
We obtained near-infrared images of the AFGL 961 region as part of our program toexplore the entire Rosette star formation complex. The JHK and H data were obtained 3 –on December 19-20, 2005, with the SOFI instrument (Moorwood, Cuby & Lidman 1998) onthe ESO New Technology Telescope at La Silla in Chile. The integration time is 2000s inthe H ∼ µ m. A 0 ′′ .6 slit was used, cuttingthrough both the exciting source and the bipolar shock structures at a position angle of 345 ◦ (north to east). Four 300s exposures of the target source were taken, which was accompaniedby a 300s exposure of the blank sky. A telluric standard star (Hip 1179 with a spectral typeof A7V, Perryman et al. 1997) was observed right after the exposures of the target. Theintegration times are 3 x 60 s for the standard star and 60 s for the sky background.Near-infrared echelle spectra covering H ×
256 pixel InSb array and has apixel scale of 0.41” × ∼
16 km s − . An internal black-body lamp was used toflat-field each spectral image, before the sky frames were subtracted from each object frame.The coadded spectral images were then wavelength calibrated using sky lines in the skyframe. The overall velocity calibration is accurate to better than 6 km s − , while velocityshifts between adjacent spectra observed along the same slit are accurate to within ∼ − . Finally, observations of HD 42807 (BS 2208; G2V, V = 6.22 mag) obtained immediatelybefore the data were used to correct for telluric absorption and to flux calibrate each spectralimage. The AFGL 961 system was observed on the night of 15 December 2006 with the KittPeak National Observatory 4-meter telescope and the Mosaic I camera (Muller et al. 1998).The pixel scale is 0.258 ′′ pixel − . Five 600 s exposures were obtained in each of the H α (k1009)and H α +16nm/[SII] (k1013) filters. The median seeing in both filters is about 0.9 ′′ . Theexposures were dithered so that the gaps between the CCDs were filled during the stackingprocess. The data were reduced with the IRAF MSCRED package in the standard manner. 4 – Medium resolution spectroscopy of AFGL 961 II was performed on the night of 22 Jan-uary 2006 with the 2.16 m telescope of the National Astronomical Observatory of the ChineseAcademy of Sciences (NAOC). An OMR (Optomechanics Rsearch Inc.) spectrograph anda Tecktronix 1024 x 1024 CCD were used. A 50 ˚A mm − grating and a 2 ′′ slit resulted ina two-pixel resolution of the spectra of 2.4 ˚A. The accuracy of the wavelength calibrationallows sampling of velocities down to 20 km s − .
3. Results
We resolve the nebulous YSO, AFGL 961 II, with our data into distinctive structures.Its spectacular appearance closely resembles a cat’s eye in our NIR colour-composite image(Fig. 1) and we thus refer to it as the Rosette Eye. It is composed of a bright young star at thecenter, an hourglass shaped diffuse nebula oriented at a position angle of 345 ◦ (from northto east), and extensive molecular emission resembling bipolar shock structures on either sideof the apparent central source. The molecular emission is the most prominent in the well-defined arc-like structures of the system. A faint star is also found in close proximity to thecentral star (see Fig. 1), but does not appear to be physically related to the excitation of theextended nebulosity. The spatial appearance of the Eye, along with the detection of strongexcesses in the near-infrared (Li & Smith, 2005), suggests the existence of an accretion diskin the perpendicular direction.At a distance of 1.39 kpc (Hensberge et al. 2000), the arc structures arch over anhourglass shaped cavity with a physical scale of ∼ ∼ H = 10 cm − (De Pree et al. 1998, Motte etal. 2001, Churchwell 2002), a pre-stellar core of this size will harbor a gas reservoir of over53.5 M ⊙ and is rich enough to give birth to a 20 M ⊙ (Li & Smith 2005) infant star accordingto theoretical models (Yorke & Sonnhalter 2002).The spatial appearance of the Rosette Eye in each of the observed bands is presentedin Fig. 2, where we see a clear bipolar nebula in J, resembling an hourglass with an openingangle of about 60 ◦ . An eye structure is superimposed; the north-west arc is more distinctivelyattached to the end of the conical nebula than is the south-east arc. There is also diffusescattered light marginally visible beyond the north-west arc. The conical nebula to the south-east possesses a sharp edge, suggesting the existence of high extinction in that direction.Diffuse extensive emission extends further to the east, pointing toward the massive binary 5 –associated with AFGL 961, probably a leakage of the stellar light over the cavity walls. Thehourglass-shaped nebula is less prominent in the H, Ks and H bands with the apparentopening angle reduced to about 45 ◦ . The arc structure in the south-east, however, brightensremarkably as the wavelength increases. It is sharp and bright in the Ks band image and ismost prominent in the H narrow-band image.In the optical narrow-band H α , the structure to the north-west resembles that observedin the J band but displays more diffuse and extended emission. The south-east lobe ap-pears to be dominated by heavy extinction and is restricted to the Eye structure. Ourmid-resolution optical spectroscopy of the exciting source shows very strong H α emission(EW=170˚A, FWHM= ∼
500 km s − ). This, along with the broad Br γ emission presentedbelow, is a good indicator of youth as also seen in other massive YSOs (Bunn et al. 1995).However, only marginal [SII] emission is detected from both the central YSO and the pho-toionized nebula. The spectroscopy with a resolution of 1.0 ˚A pixel − yields no velocitydifference between the bipolar lobes. This suggests a low velocity of at most a few tens ofkm s − of the outflowing gas, in accord with the general properties of outflows associatedwith massive protostellar objects (Churchwell 2002; Henning et al., 2000).Given the optical spectroscopy results, the [SII] image closely represents continuumemission from the Eye. The derived continuum-subtracted H α emission shows a small HIIregion excited by the UV ionizing photons from the YSO and faint net emission from thenorth-west arc, indicating marginal ionization in the shell. The small HII region is confinedby the bipolar cavity, which is less than 0.1 pc in size. Here the UV ionizing photons areattributed to shocked accretion from the proposed disk/envelope in the orthogonal direction,which feeds the YSO continuously till the final dissipation of the circumstellar materials.The morphology of the Eye matches well the transient phase in massive stellar evolutionpresented by Keto 2007, where an hourglass-shaped photon-dissipation region first formsbefore it evolves into a full-blown HII region. The excitation of the ionized nebula, however,suggests a spectral type earlier than B2 for the exciting star, commensurate with its positionon the color-magnitude diagram in the NIR (Li & Smith 2005).Archived Spitzer IRAC and MIPS imaging data on the AFGL 961 region, which traceshot dust distribution, indicate that AFGL 961 II, the exciting source of the Eye, sits onthe edge of a lane of dust with the highest extinction. The conical structure in the south-east encounters heavy dust extinction. This explains why the bipolar nebula is observedin the optical as a fan shaped nebula extending toward the north-west (Li & Smith 2005).The MIPS 24 µ m imaging, on the other hand, indicates a dumbbell shaped nebula thatencompasses the bipolar cavity detected in the NIR, which is devoid of dust emission andimplies the dissipation or evacuation of dust by the ionized flows. With a moderate extinction 6 –of ∼ line emission often arises after collisional excitation within shock waves inassociation with mass ejections from extremely young stars. This usually yields relativelyhigh radial velocities and strong excitation of the lower vibrational levels. To test this hy-pothesis, we performed deep Ks band spectroscopy with a slit position cutting through boththe arc structures and the central YSO. The spectrum of the exciting source shows Br γ (EW=10.8˚A, FWHM=29˚A) superimposed on a featureless rising continuum towards longerwavelengths. However, the H emission from the shell displays no sign of collisional excita-tion. Line emission originates from very high vibrational levels of H (Fig. 3, upper panel).The highest level present is the 9–7 Q(3) at 2.100 µ m, originating 42,462 K above the groundstate. The spectroscopic data demonstrate distinctly that the gas is purely fluoresced withthe H line strengths corresponding exactly to an energy level cascade. This, however, isin agreement with theoretical predictions that UV pumped fluorescence should be a com-mon phenomenon, especially on surfaces of molecular clumps illuminated by young massivestars (Gatley 1987). AFGL 961 II thus represents the first detection of pure fluorescentradiation directly associated with massive protostellar objects.The fluorescence origin of the line emission is confirmed by our simulation based onthe observed line flux ratios (Smith, Li & Gredel et al. in prep.). Furthermore, the echellespectrogram obtained by UKIRT discloses no radial velocity motion in the shell to the limitsof our measurements ( ∼ − ) (Fig. 3, lower panel).
4. Summary & discussion
This study substantiates that the exciting source of the Rosette Eye, AFGL 961 II,has just finished its original collapse, that UV radiation from shocked massive accretionhas recently turned on and that ionized stellar winds have begun to emerge in the polardirections. The shell left by the original collapse is now prepared to face the UV ionizationfrom the newly born star. We suggest that the Eye may closely correspond to a transientphase immediately preceding that of the famous Orion OMC-1 outflow (Stone et al. 1995).In this scenario, the arcs will be subjected to fluid instabilities as an ensuing fast wind of lowdensity emerges, which leads to the so-called fireworks as the shell gas is driven out in theform of dense bullets (McCaughrean & Mac Low, 1997). The results presented in this papercorroborate the recent onset of the formation of a massive star in the ridge of the RMC,which is in a key transient phase of its emergence from the natal cloud and the developmentof the HII region. 7 –We are grateful to the referee, Robert Gehrz, for the many helpful comments and sugges-tions made. This work was supported by INTAS grant 4838 and funding from the NationalNatural Science Foundation of China through grant 10503006.
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This preprint was prepared with the AAS L A TEX macros v5.2.
Line wavelength flux flux Line wavelength flux flux ( µ m ) South North ( µ m ) South North1 − ( ) .
033 38 .
60 20 .
70 4 − ( ) . < . < . − ( ) .
041 10 .
30 7 .
04 2 − ( ) .
154 18 .
20 10 . − ( ) .
065 3 .
89 3 .
49 9 − ( ) .
172 3 .
36 3 . − ( ) . < . < .
00 3 − ( ) .
201 16 .
50 10 . − ( ) ∗ .
073 4 − ( ) ∗ . − ( ) .
073 20 .
30 14 .
80 8 − ( ) .
210 9 .
91 6 . − ( ) .
084 3 .
09 3 .
36 1 − ( ) .
223 69 .
40 32 . − ( ) .
100 5 .
59 4 .
39 2 − ( ) .
247 43 .
70 30 . − ( ) . < . < . − ( ) .
253 10 .
20 6 . − ( ) † .
121 10 .
35 5 .
00 4 − ( ) .
268 4 . < . − ( ) † .
121 127 .
65 61 .
60 3 − ( ) .
286 10 .
00 8 . − ( ) .
127 10 .
30 5 . Units of 10 − W m − . The 1 σ flux measurement uncertainty is 2 × − W m − and 3.0 × − W m − beyond 2.25 µ m. The 12-9 O(3) flux upper limit is subject to considerableerror due to the location near a strong line. ∗ – Line blended; no decomposition possible. † – relative line fluxes decomposed from echelledata in approximately the same regions. 10 –Fig. 1.— Close-up view of the Rosette Eye. North is up and East is to the left. The colorcomposite with a size of 19 ′′ .0 × ′′ .0 was compiled based on the NTT J (blue), H (green)and H (red) observations. The image displays both sharp outer features and inner diffuseemission. The northern rim is considerably redder and more distant from the central object.Note that the diffuse fan is stronger in the south with brighter outer edges, suggestive of aconical shell structure in three dimensions. 11 –Fig. 2.— Greyscale images of the AFGL 961 II field in J (upper-left), H (upper-middle), Ks(upper-right), 2.12 µ m (lower-left), H α (lower-middle) & MIPS 24 µ m (lower-right). Northis up and East is to the left. The morphology of the Eye changes dramatically betweenbands. In the J band, it resembles an hourglass with an eye structure superimposed. In H,Ks and H , the Eye is prominent. In H α , it indicates predominantly a fan shaped nebulathat extends to the NW and higher extinction in the opposite direction. The outflow cavityis encompassed by a dumbbell shaped dusty bubble as disclosed by the MIPS 24 µ m image. 12 –Fig. 3.— NTT Ks-band spectra of the arc structures (upper panel) with the identifiedmolecular hydrogen lines indicated. It is obvious that line emission originating from veryhigh vibrational levels of H are present. The highest level present is the 9–7 Q(3) at2.100 µ m. It is thus conclusive that there is no indication of any shock emission at all.Instead, the sharp features are produced by an exemplary case of fluoresced H2