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HST/NICMOS Observations of a Proto-Brown Dwarf Candidate The A&A 433, L33–L36 (2005) Astronomy DOI: 10.1051/0004-6361:200500098 & c ESO 2005 Astrophysics Editor HST/NICMOS observations of a proto-brown dwarf candidate the 1,2,3 1,4,5 1 6 1 1 to D. Apai ,L.V.Tóth , T. Henning ,R.Vavrek ,Z.Kovács ,andD.Lemke 1 Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany e-mail: [email protected] 2 Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA Letter 3 NASA Astrobiology Institute 4 Department of Astronomy of the Loránd Eötvös University, Pázmány Péter sétány 1, 1117 Budapest, Hungary 5 Konkoly Observatory of the Hungarian Academy of Sciences, PO Box 67, 1525 Budapest, Hungary 6 Astrophysics Missions Division, ESTEC, Keplerlann 1, Norwijk 2201, The Netherlands Received 6 December 2004 / Accepted 18 February 2005 Abstract. We present deep HST/NICMOS observations peering through the outflow cavity of the protostellar candidate IRAS 04381+2540 in the Taurus Molecular Cloud-1. A young stellar object as central source, a jet and a very faint and close (0.6) companion are identified. The primary and the companion have similar colours, consistent with strong reddening. We argue that the companion is neither a shock-excited knot nor a background star. The colour/magnitude information predicts a substellar upper mass limit for the companion, but the final confirmation will require spectroscopic information. Because of its geometry, young age and its rare low-mass companion, this system is likely to provide a unique insight into the formation of brown dwarfs. Key words. binaries: close – planetary systems: protoplanetary disks – stars: formation – stars: low-mass, brown dwarfs – stars: pre-main sequence 1. Introduction (Klein et al. 2003) are too uncertain to constrain possible disk truncation and therefore cannot exclude the ejection scenario. The debate on the origin of brown dwarfs has preceded their Direct observations of the youngest phases of substellar ob- discovery (Rebolo et al. 1995): their small masses poses seri- jects, termed as proto-brown dwarfs, will be crucial in evaluat- ous difficulty to a stellar-like formation scenario through the ing the formation scenarios. Recent imaging by Haisch et al. collapse of a molecular cloud core. Now, with hundreds of (2004) and Duchêne et al. (2004) identified faint, possibly sub- known brown dwarfs, the debate is still not settled: the most stellar companion candidates to protostellar systems, but some widely discussed formation scenarios include gravitational col- of them might be background stars. To ensure the very young lapse in highly turbulent cloud cores (e.g. Padoan & Nordlund age of the proto-brown dwarfs, they should be observed within 2004), early ejection from unstable multiple systems (Sterzik the massive protostellar envelope, possible only in exceptional & Durisen 1998; Reipurth & Clarke 2001; Umbreit et al. 2004) cases. and photoevaporation of protostars (Whitworth & Zinnecker We present here observations of the first such system, 2004). Only few traces of the formation process survive the IRAS 04381+2540, located in TMC-1 in the Taurus star- early evolution of these objects. Current studies focused on forming region with the discovery of a proto-brown dwarf can- disk indicators, signs of accretion, binarity and velocity dis- didate. Based on its spectral energy distribution and bolometric persion. We learned that brown dwarfs are often surrounded by temperature, IRAS 04381+2540 has been classified as a Class I warm circumstellar material (e.g. Jayawardhana et al. 2003), object (Hogerheijde & Sandell 2000). However, estimates of with masses of ∼MJup (Klein et al. 2003) which they main- the stellar mass (M = 0.3 ± 0.1 M, Brown & Chandler 1999) tain over timescales similar to stars (Sterzik et al. 2004), and show that it is comparable to the envelope mass (M = 0.18 M, in one case it could be shown that the dust forms a disk Young et al. 2003) indicating that the object is still in its main (Pascucci et al. 2003). Because gravitational ejection leaves accretion phase (Motte & André 2001). IRAS 04381+2540 sufficient material in the inner disk (Bate et al. 2002) to pro- drives a molecular outflow (Brown & Chandler 1999), with duce infrared excess emission and accretion signs, the ejec- an inclination of i ≈ 50◦ (the northern part tilted toward the tion embryo hypothesis remains a possible formation scenario. observer, Chandler et al. 1996). Through the evacuated out- The masses derived from millimetre-wavelength observations flow cone we see into the dense molecular cloud core in which Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:200500098 L34 D.Apaietal.:HST/NICMOS observations of a proto-brown dwarf candidate Table 1. Log of the infrared observations and results of the NICMOS photometry. The estimated photometric error is less than 10% for all bands. In the F212N band only an upper limit can be reliably established due to the low contrast between the object and the background nebulosity. Here, the use of a smaller aperture reduces the contamination from the nebulosity and places a stricter upper limit. The F160W filter also includes [Fe II] line emission at 1.644 µm. Editor Instrument Filter Date Field of Exp. time Flux of NIR A Flux of NIR B Note DD/MM/YY view [s] [µJy] [mag] [µJy] [mag] the HST/NICMOS F160W 20/12/97 20 × 19 1280 511 15.8 38 18.6 1.4–1.8 µm / / / × µ to HST NICMOS F205W 07 01 98 19 19 256 6295 12.6 363 15.7 1.75–2.35 m HST/NICMOS F212N 07/01/98 19 × 19 160 7711 12.3 <1070 >14.5 2.121 µm, H2 HST/NICMOS F212N 07/01/98 19 × 19 160 ≤351 0.13 aper. Letter IRAS 04381+2540 is embedded, making it an exceptional target for studies of very early stellar evolution. 4 2. Observations and data reduction 2 S ND / CO 0 E We used archival HST NICMOS observations to study S C R IRAS 04381+2540 at small spatial scales. The main parame- A ters of the observations are summarised in Table 1. -2 The data have been successfully pipeline-processed and re- NIR B quired only minor additional reduction work, which was car- -4 NIR A ried out by simple IDL scripts. The images have been bad 4 3 2 1 0 -1 -2 -3 -4 pixel filtered and the central columns bias-corrected in the stan- ARC SECONDS h m s o dard fashion. The final mosaic has been composed by cross- CENTRE: R.A. 04 41 12.70 DEC +25 46’38."7 (2000) correlating the individual pointings. In order to enhance the Fig. 1. High-resolution near-infrared view of IRAS 04381+2540 by fainter details around the central point source we subtracted HST/NICMOS in the broadband F160W filter. This image reveals a + synthetic point spread functions (PSF) calculated by using the binary young stellar object (NIR A B) and a collimated jet. Tiny Tim 6.1 (Krist & Hook 1997). The scaling factors have been obtained by comparing the integrated counts in 3-pixel- the spacecraft’s pointing accuracy, typically better than 0. 5. radii apertures placed on the simulated and observed peaks. All NICMOS images show NIR A as a point source at a res- Aperture photometry has been performed on all images in olution of 0. 15. The secondary source NIR B is located at an identical way. We used the IDL adaptation of the DAOPHOT 0. 59 east and 0. 15 north of NIR A. The source is unresolved in routine to integrate the counts in 6-pixel-radius apertures cen- all three NICMOS images, while in the F160W image a pos- tered on the central point source. In order to obtain reliable sible marginal, faint extension is seen at the brightness level photometry also for the fainter, secondary point source we car- of the surrounding nebula, possibly a faint scattered-light en- ried out PSF photometry by subtracting simulated PSFs. By velope of NIR B. The photometry results are summarized in minimizing the subtraction residuals we measured the point Table 1. The deepest F160W image also reveals the presence source’s brightness without significant contribution from the of a ∼3. 3 long, narrow line of intense emission, which we surrounding nebulosity. In the case of the F212N filter, due to identified as a highly collimated jet. While the northern end the faintness of the secondary point source compared to the of the jet vanishes in the background noise about 5 from nebula, we could only estimate upper limits from the aper- NIR A, its southern end is overshined by the reflection nebula ture photometry. Here, we also applied a second, small 3 × 3 at about 1.2 from NIR A. The jet’s direction points to NIR A pixel aperture to suppress the contamination from the nebula identifying its driving source. and thus obtained a stricter upper limit. 4. Discussion 3. Results The detection of the collimated jet proves that NIR A, its driv- As shown in Fig. 1 the NICMOS image reveals a detailed ing source, is still in the early, accreting phase of its evolu- view of the reflection nebulosity including two point sources tion. By assuming an inclination of i = 50◦ for the jet its (IRAS 04381+2540 NIR A and IRAS 04381+2540 NIR B) visible length is ∼1000 AU. As often observed, the jet in and a long near-infrared jet at the heart of IRAS 04381+2540. IRAS 04381+2540 is accompanied by a molecular outflow In the following, we will refer to the brighter, primary point (Chandler et al.
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