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Home All issues Volume 526 (February 2011) A&A, 526 (2011) A55 Full HTML
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Issue
A&A
Volume 526, February 2011
Article Number A55
Number of page(s) 3
Section Stellar structure and evolution
DOI https://doi.org/10.1051/0004-6361/201015289
Published online 21 December 2010

© ESO, 2010

1. Introduction

Tremendous efforts have been made over the past decade to study the ultracool end of the local field mass function. To date, several hundred methane dwarfs have been discovered as a result of sensitive surveys such as the 2MASS (Skrutskie et al. 2006), DENIS (Epchtein et al. 1997), UKIDSS (Lawrence et al. 2007) and CFHTBDS (Delorme et al. 2008) near-infrared surveys, and the SDSS survey (York 2000). Because methane dwarfs cannot sustain fusion in their core, their temperature drops continuously with time. A number of studies have therefore searched for ever cooler objects, representative of the proposed Y spectral class (Delorme et al. 2008; Burningham et al. 2008; Warren et al. 2007), and encountered a few objects with effective temperatures of only  ≈600 K. Most recently, Lucas et al. (2010) reported the discovery of an ultracool dwarf (UGPS J072227.51-054031.2) with an effective temperature of only Teff  ≈  500 K, making it the coolest brown dwarf known to date. They classify it as a T10, at the boundary between T and Y dwarfs. Because methane dwarfs are frequently found in multiple systems (Burgasser et al. 2003; Goldman et al. 2008), we decided to perform high angular resolution observations of the target to look for close companions. Resolving the source into a multiple system would further extend the domain of absolute magnitude for known ultracool dwarfs, and might significantly revise the estimated luminosity and mass of the primary component.

2. Observations and data reduction

The source UGPS J072227.51-054031.2 was observed with NACO, the Very Large Telescope (VLT) adaptive optics system (Lenzen et al. 2003; Rousset et al. 2003) and its laser guide star on the 2010 April 12 as part of program 285.C-5004. A set of deep images were obtained in H and Ks where the compromise between the target’s luminosity and the AO performances is the best.

At the time the observations were prepared, Lucas et al. (2010) had ruled out the presence of companion at distances larger than 3″ using deep seeing limited images. We therefore decided to limit our study to the immediate vicinity of the target, and used a 512  ×  512 pixels window with the S27 camera, providing a final field of view of  ≈14  ×  14″. This setting allows us a faster read-out and an optimized use of the cube mode. In cube mode, a data-cube with each individual DIT frame is saved, allowing for a careful frame selection and optimized weighting in the post-processing and co-addition. A set of 208 and 240 4 s individual images were acquired in H and Ks, respectively. The target was too faint to be used as reference star for NACO’s visible or near-infrared wavefront sensors. We therefore used the laser guide star for the high-order corrections, and the R  ~ 15.8 mag star 2MASS J07222849-0540377 located at  ~19″ from the science target for tip-tilt correction (see Fig. 1). The ESO seeing monitor reports a DIMM seeing between 05  ≲  σ  ≲  10, a coherence time of the atmospheric turbulence between 5  ≲  τ0  ≲  10 ms, while the sky transparency was classified as photometric.

thumbnail Fig. 1

UKIDSS J-band image showing the field of view of the NACO images, and the tip-tilt reference star. The scale is indicated. North is up and east is left.

The individual images were successively dark-subtracted and flatfielded using calibration frames obtained as part of the standard calibration sequence. The Strehl ratio was then measured on the target for each individual image of the cube by fitting a Moffat function to the target’s point spread function (PSF), and dividing the corresponding normalized peak to the normalized peak of a theoretical VLT diffraction limited PSF. The individual images were then co-added weighted by their respective Strehl ratio. The final Strehl ratio is  ≈12% in both the H and Ks image. Figure 2 shows the final images.

Table 1

Astrometry and photometry of the sources present in the NACO images.

thumbnail Fig. 2

NACO H (left) and Ks (right) images of UGP J072227.51-054031.2. The lower panels show a zoom on the target. The scale and orientation are indicated.

We extracted the photometry of all sources brighter than the 3-σ local noise using standard aperture photometry procedures within IRAF1. The zeropoints were computed using the H and Ks photometry of four sources with a counterpart in the UKIDSS survey. The final uncertainties are largely dominated by the zeropoint uncertainties, computed as the 1-σ standard deviation between the four measurements.

thumbnail Fig. 3

Limit of sensitivity of our observations, computed as the 3-σ standard deviation of the radial profile of the PSF. The projected distance scale assuming a distance of 4.1 pc is indicated. The luminosity of UGPS J072227.51-054031.2 is indicated with a star.

3. Analysis and conclusions

No obvious close companion is detected around the target. A careful PSF subtraction using the two brightest sources present in the field produces clean residuals. Figure 3 shows the 3-σ limit of detection of the H and Ks images, computed using the standard deviation of the radial profile of the PSF. The source is slightly elongated towards the tip-tilt reference star, and Fig. 3 represents an average of the limit of sensitivity over all azimuthal directions. The observations allow us to rule out the presence of a companion brighter than H  ≲  19.2 mag and K  ≲  19.1 mag at separation larger than 50 mas (0.205 AU at 4.1 pc), and H  ≲  21.2 mag and Ks  ≲  20.2 mag at separation larger than 01 (0.41 AU at 4.1 pc). A total of seven additional sources are present in the image. Table 1 gives an overview of their astrometry and photometry. All but one are detected in the J-band image dated 2006 November 28 of Lucas et al. (2010), and are located at the same absolute position (within 01) in our NACO images while the target has moved by more than 3″ between these two epochs. We therefore rule out these six sources as possible companions based on their inconsistent proper motion. The last source is significantly redder than the target (H − Ks = 0.36 mag). As a cooler companion would be expected to be bluer, we reject this last source as a possible companion.

1

IRAF is distributed by National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract with the National Science Foundation.

References

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All Tables

Table 1

Astrometry and photometry of the sources present in the NACO images.

In the text

All Figures

thumbnail Fig. 1

UKIDSS J-band image showing the field of view of the NACO images, and the tip-tilt reference star. The scale is indicated. North is up and east is left.

In the text
thumbnail Fig. 2

NACO H (left) and Ks (right) images of UGP J072227.51-054031.2. The lower panels show a zoom on the target. The scale and orientation are indicated.
In the text
thumbnail Fig. 3

Limit of sensitivity of our observations, computed as the 3-σ standard deviation of the radial profile of the PSF. The projected distance scale assuming a distance of 4.1 pc is indicated. The luminosity of UGPS J072227.51-054031.2 is indicated with a star.
In the text

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