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THE ASTRONOMICAL JOURNAL, 116 : 440 õ 443, 1998 July
( 1998. The American Astronomical Society. All rights reserved. Printed in U.S.A.

WIDE FIELD PLANETARY CAMERA 2 OBSERVATIONS OF PROXIMA CENTAURI : NO EVIDENCE OF THE POSSIBLE SUBSTELLAR COMPANION DAVID A. GOLIMOWSKI1 AND DANIEL J. SCHROEDER2
Received 1998 March 10 ; revised 1998 March 25

ABSTRACT Two-epoch observations of Proxima Centauri using the Hubble Space T elescope (HST ) Wide Field Planetary Camera 2 (WFPC2) are reported. Exposures of 10 and 40 s were recorded through the F1042M ïlter (j B 1 km), permitting examination of the circumstellar region beyond 0A 9 from .0 c Proxima Cen. No evidence of a substellar companion within 0A 5 of Proxima Cen is seen, which .8 counters the recently reported detection of a faint feature by Schultz et al. using HST îs Faint Object Spectrograph (FOS). A feature in the WFPC2 images having a relative brightness and a separation comparable to those of the FOS feature would have been detected with a signal-to-noise ratio of D22. Moreover, if the FOS feature were a substellar companion, it should have appeared in our F1042M images to be about 3.7 mag fainter than Proxima Cen. Inspection of deep WFPC2 images of Proxima Cen through three ïlters indicates that the FOS feature is not a background object. Local enhancements of WFPC2îs point-spread function suggest a possible instrumental origin for the FOS feature, but the singularity and apparent motion of the FOS feature complicate this notion. Key words : stars : individual (Proxima Centauri) õ stars : low-mass, brown dwarfs
1

. INTRODUCTION

Proxima Centauri (Gliese 551, LHS 49 ; V \ 11.09 ; spectral type M5.5 V) is the star closest to the Sun. Its proximity (1.3 pc) and low mass (0.1 M ) have made it a popular _ target for seeking extrasolar planets or substellar companions. Several search techniques have been employed in this quest, including astrometry (Kamper & Wesselink 1978 ; Benedict et al. 1995, 1998), radial velocity (Hatzes et al. 1996), photometric variability (Benedict et al. 1993), nearinfrared imagery (Jameson, Sherrington, & Giles 1983), and near-infrared speckle interferometry (Leinert et al. 1997). The results of these searches have greatly constrained the physical and dynamical characteristics of any bodies orbiting Proxima Cen. For example, Leinert et al. (1997) placed a limit of 3 to 5 K-band magnitudes below the empirical end of the main sequence (M B 10) for brown dwarf companK ions located 1 to 10 AU from Proxima Cen. Benedict et al. (1998) recently reported astrometric upper limits of 1 Saturn mass for companions with periods P [ 400 days (orbital radii r [ 0.5 AU) and 1 Jupiter mass for companions with P \ 40 days (r \ 0.1 AU). Countering the trend of null results, Schultz et al. (1998, hereafter S98) reported the detection of a possible substellar companion to Proxima Cen in coronagraphic images obtained with the Hubble Space T elescope (HST ) Faint Object Spectrograph (FOS). Unïltered and reconstructed FOS images obtained 104 days apart showed two bright featuresõone in each imageõlocated approximately 0A .3 from Proxima Cen at position angles separated by about 65¡. Although accurate ÿux measurements were impossible, S98 estimated that the feature is about 7 mag fainter than Proxima Cen in the eective bandpass of the FOS red detector. Unable to explain the feature as an instrumental artifact or a chance alignment with a background star, S98
õõõõõõõõõõõõõõõ 1 Department of Physics and Astronomy, Johns Hopkins University, 3400 North Charles Street, Baltimore, MD 21218. 2 Department of Physics and Astronomy, Beloit College, 700 College Street, Beloit, WI 53511.

suggested that the feature may be a brown dwarf in a highly eccentric, long-period orbit. They determined that such an object could have eluded astrometric detection. We are presently engaged in a search for very low mass (VLM) companions to nearby stars using HST îs Wide Field Planetary Camera 2 (WFPC2) (Schroeder & Golimowski 1996). We have obtained direct images of Proxima Cen through WFPC2îs F1042M ïlter (j B 1 km) at two epochs following the observations of S98. c Our images reveal the circumstellar region beyond 0A 9 from Proxima Cen, which .0 includes the ïeld of view of the FOS coronagraphic images. In this paper, we present these WFPC2 images and report no evidence of the FOS feature reported by S98. We discuss the sensitivity of our search and the likelihood of imaging the FOS feature if it were a substellar companion. Finally, we discuss the possibility that the FOS feature may be an instrumental eect, such as a local enhancement of the point-spread function (PSF).
2.

OBSERVATIONS AND DATA REDUCTION

Observations of Proxima Cen were performed on UT 1997 March 30 and April 20 using WFPC2. These observations occurred within 6 months of the last FOS observation by S98. Proxima Cen was acquired at the approximate center of the Planetary Camera (PC), providinga34A ] 34A ïeld of view (FOV) centered on the star. The telescope was rolled counterclockwise by 44¡ between visits to Proxima Cen, causing a like rotation of the FOV. During each visit, two 10 s exposures and ïve 40 s exposures were recorded at a gain of 7e~ DN~1 through the F1042M ïlter (j \ 1.02 km, *j \ 0.04 km). Although the detective c quantum efficiency of WFPC2 through F1042M is low (0.36% peak), the contrast in brightness between mainsequence stars and their putative VLM companions is smallest in this bandpass. The images were reduced by the HST data calibration pipeline (Leitherer et al. 1995). Recalibration of the images with more contemporary calibration data was not necessary for the purposes described in this paper. The images of each exposure set were combined using a pixel rejection 440


10 sec
N

40 sec
N E

1997 March 30

E

E

N

E

N

Model
0.25"

1997 April 20

"Beads"

FIG. 1.õPC images of Proxima Cen through the F1042M ïlter. The top and middle panels show the 10 and 40 s exposures obtained on UT 1997 March 30 and April 20, respectively. The logarithms of the pixel intensities are displayed to reduce image contrast. The saturated pixels near the image centers have been blackened to demark the observable circumstellar region. The bottom panels show model images of the PCîs PSF through F1042M. These panels display the same model suitably scaled to mimic the level of exposure of the actual PC images above. Each panel depicts a 1A ] 1A section of the full .7 .7 PC image centered on the star. This FOV is the same observed by S98 using the large barred aperture of FOS. No evidence of the FOS companion candidate is seen. The beadlike features on the bright fourth Airy ring lie D0A from the image center and are D6.5 mag fainter than the image core. .4


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GOLIMOWSKI & SCHROEDER

Vol. 116

algorithm that eliminates artifacts, such as cosmic rays, that deviate by at least 3 p from the local mean.
3

. RESULTS

Figure 1 shows the 10 and 40 s images of Proxima Cen obtained on UT 1997 March 30 (top) and UT 1997 April 20 (middle). The logarithms of the pixel intensities are displayed to reduce image contrast. Each panel depicts a 1A ] 1A section of the full PC image centered on the star. .7 .7 Thus, the panels show the same FOV around Proxima Cen observed by S98 using the large barred aperture of FOS. The saturated pixels near the center of each image have been blackened to demark the observable circumstellar region. Using an image scale of 0A04554 pixel~1 for the PC . (Holtzman et al. 1995), we compute the radial extent of saturated pixels to be 0A 9 and 0A 0 for the 10 and 40 s .0 .2 exposures, respectively. By comparison, the half-width of the FOS occulting bar used by S98 was 0A15. . Because the images of Proxima Cen are saturated, conventional aperture photometry cannot be used to measure the starîs ÿux through F1042M. An estimate of this ÿux can be made, however, by comparing the unsaturated region of the PSF with that of another well-exposed but unsaturated star. Golimowski et al. (1998) obtained unsaturated F1042M images of the M1 V star Gliese 229A. By suitably scaling the azimuthally averaged PSF of Proxima Cen to match that of Gl 229A, we estimate a WFPC2 magnitude of m B 6.0 for Proxima Cen. 1042 bottom panels of Figure 1 show model images of the The PCîs PSF through the F1042M ïlter. Each panel displays the same model image suitably scaled to mimic the level of exposure of the actual PC images in the same column of panels. The models were generated in the manner described by Schroeder & Golimowski (1996) for a star centered on the contiguous corners of four pixels. The PSFs have been convolved with a 9 ] 9 pixel kernel that simulates the pixel response function of the WFPC2 detectors (Burrows et al. 1995). Aberrations caused by mirror-polishing errors are not included in the models, but their eect on the PSF is not large for bandpasses as red as F1042M (Krist & Burrows 1995). The good match between the images of Proxima Cen and our single-PSF models indicates that only one stellar source has been imaged in the FOV. Each pointlike feature in the PC images has a counterpart in the model PSF. (Subtle dierences between the actual and model PSFs are attributable to subpixel osets of the image centers. Note that the images obtained on UT 1997 April 20 are nearly centered, like the models, on the contiguous corners of four pixels.) None of the pointlike features move as the FOV rotates between visits, so these features cannot be images of astronomical sources. Of the many features present in the PCîs complex PSF, one deserves special mention. The bright annulus of radius D0A (9 pixels), visible in both the real and model images, is .4 the enhanced fourth Airy ring that is characteristic of a circular aperture with a central obscuration ratio of 0.41 (Schroeder 1987, p. 182). This diraction eect is illustrated in Figure 2. The surface brightness of the fourth ring is modulated by diraction from the primary- and secondarymirror support structures, which produces compact "" beads îî that are approximately 6.5 mag fainter than the core of the PSF. These "" beads îî are useful benchmarks for

FIG. 2.õRadial plot of the theoretical Airy diraction pattern for a circular aperture with central obscuration ratio of v \ 0.41 and a monochromatic wavelength of 1.02 km. Binning this pattern to the pixel resolution of the PC produces a PSF similar to that shown in Fig. 1. Note the prominence of the fourth diraction ring (marked "" 4 îî) at a distance of D0A (9 pixels). Modulation of this ringîs intensity by HST îs mirror.4 support structures causes the beading eect seen in Fig. 1. Similar "" beads îî are present in the FOS PSF at locations governed by v \ 0.33 and the eective wavelength of the FOS image.

determining the sensitivity of our images to substellar companions. Our PC images of Proxima Cen recorded 21 days apart exhibit no evidence of the possible companion noted by S98 from their coronagraphic FOS images. The 10 p threshold for detecting point sources located 0A from Proxima Cen .3 (the average separation of the FOS feature) is M B 17.6 1042 for the 40 s images. This threshold is 1.3 absolute magnitudes fainter than the brown dwarf Gl 229B in this bandpass (Golimowski et al. 1998). A feature in these images having a relative brightness and a separation comparable to those of the FOS feature would have been detected with a signal-to-noise ratio of D22.
4.

DISCUSSION

Using count rates derived from model brown dwarf spectra, S98 concluded that the FOS feature, if real, is consistent with a brown dwarf that is about 6 times more luminous than Gl 229B (Nakajima et al. 1995) in the eective bandpass of the unïltered FOS red detector. To investigate this claim and its implications, we have listed in Table 1 the measured optical and near-infrared magnitudes of several known late-type objects. Although the magnitudes listed in Table 1 come from dierent photometric systems, the colors of such red objects in these systems are similar. Using the measured FOS count rate from the M6 V star Wolf 359, S98 estimated a Gunn magnitude of r B 15.8 ^ 0.5 for the FOS feature. Assuming the feature shares Proxima Cenîs parallax of 0A77233 (ESA 1997), we . compute M B 20.2 ^ 0.5. Thus, the feature is about 17 r times less luminous than the M9 V star LHS 2065 (Bessell 1991) and about 60 times more luminous than the brown dwarf Gl 229B (Golimowski et al. 1998) at R-band wave-


No. 1, 1998

WFPC2 OBSERVATIONS OF PROXIMA CEN
TABLE 1 OPTICAL AND NEAR-INFRARED PHOTOMETRY OF VERY LOW MASS OBJECTS Object Spectral Type M1 V M5.5 V M7 V M9 V Brown dwarf V, M 8.12,a 11.09,a 16.80,a 18.74,d .. V 9.31b 15.53b 17.75c 19.08c . R, M 7.16,a 9.44,a 14.65,a 16.74,d 23.41,e R 8.35b 13.88b 15.60c 17.09c 24.60b I, M 6.11,a 7.44,a 12.24,a 14.54,d 19.57,e I 7.30b 11.88b 13.19c 14.89c 20.76b m ,M 1042 1042 5.48,e 6.67b 6.0,f 10.4b ... ... 15.18,e 16.37b J, M 4.98,a 5.28,a 9.77,a 11.30,d 14.2,g J 6.17b 9.72b 10.72c 11.65c 15.4b

443

Gl 229A ........... Proxima Cen ...... Gl 644C ........... LHS 2065 ......... Gl 229B ........... a b c d e f g

Photometry from Leggett 1992. Parallax from ESA 1997. Parallax from van Altena, Lee, & Hoffleit 1995. Photometry from Bessell 1991. Photometry from Golimowski et al. 1998. Photometry from this paper. Photometry from Matthews et al. 1996.

lengths. This result is a necessary condition for a substellar classiïcation of the FOS feature, but it is not a sufficient one, as we now show. Because Gl 229B is the only VLM object so far measured through F1042M, it is impossible to predict accurately the brightness of the FOS feature in this bandpass. However, we may estimate this quantity by interpolating bilinearly between the magnitudes listed in Table 1 for Proxima Cen, B 9.7. LHS 2065, and Gl 229B. Doing so, we obtain m 1042 Thus, the FOS feature should have appeared in our F1042M images to be D3.7 mag fainter than Proxima Cen and D3.2 mag brighter than the "" beads îî on the fourth Airy ring. No such feature is seen in the PC images recorded during either visit. Therefore, we conclude that the FOS feature cannot be a VLM or brown dwarf companion to Proxima Cen. If the FOS feature is not a substellar companion, then what is it ? The sensitivity limit described at the end of ° 3 discourages the notion that the feature shines by neutral scattering of starlight from Proxima Cen. Moreover, deep (400 s) F1042M exposures of Proxima Cen recorded during both WFPC2 visits show no background objects at the projected locations of the FOS aperture during S98îs observations of Proxima Cen. Earlier R- and I-band PC images of Proxima Cen from our program also reveal no ïeld sources in the corresponding locations of the FOS aperture (S98). Thus, we are unable to provide a viable interpretation of the FOS feature as an image of a known type of astronomical object. Because FOS was rarely used for scientiïc imaging, its PSF was formally characterized only once (Koratkar 1996). Then, the white-light PSF of the red detector exhibited a linear feature that was oriented nearly perpendicularly to the occulting bar. This linear feature was attributed to

"" diraction spikes convolved with the COSTAR [Corrective Optics Space Telescope Axial Replacement] mirrors in the PSF. îî Unfortunately, the positional or temporal behavior of the FOS PSF was never studied (E. Smith 1998, personal communication), so the possibility that S98îs feature is an artifact of a variable PSF cannot be rigorously assessed. S98 discounted the possibility of an instrumental artifact because they observed no similar features near the other target stars in their survey. However, those stars (Wolf 359, LHS 292, Gl 293, GJ 1245AC, and Wolf 424AB) are 1.5 õ 4.5 mag fainter than Proxima Cen (Leggett 1992), so compact features of their PSFsõlike the "" beads îî described in ° 3õwould probably have been fainter than S98îs detection threshold of V B 20. Given the lack of corroborating evidence in our PC images and the understudied nature of the FOS PSF, we do not support S98îs proposition that the FOS feature is a substellar companion to Proxima Cen. Although we cannot deïnitively describe the nature of the FOS feature, neither can we explain it as an astronomical phenomenon. Local enhancements of the PCîs PSF suggest a possible instrumental origin for the FOS feature, but the singularity and apparent motion of the FOS feature complicate this notion. Certainly, additional observations will help to settle this issue. The authors thank Peg Stanley for her assistance in implementing the observations. We also thank Ed Smith for a helpful discussion of the FOS PSF. Support for this work was provided by NASA grants NAG 5-1617 and NAG 5-1620. The Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.

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