A new class of warm debris disks?Rachel Smith, rs@roe.ac.uk, Institute for Astronomy; Mark Wyatt, wyatt@roe.ac.uk, UKATC.

Abstract

The few Vega-type stars whose dusty debris disks have been resolved show this dust lies in cool Kuiper belt-like rings. However, roughly half of all debris disk candidates exhibit little or no cool dust, since their emission peaks at around 25um. We have conducted a mid-IR observing programme of stars identified as warm debris disk candidates in the IRAS catalogue using the TIMMI2 instrument on the 3.6m telescope at La Silla. The results confirm the presence of disks around several stars.

Introduction

Over the last 15 years, analysis of the IRAS database has demonstrated that there are in excess of 300 nearby main-sequence stars that have dusty disk around them. Examination of the SED’s of the best studied cases show that this dust is cool and so lies in Kuiper belt-like regions; this has been confirmed where the disks have been resolved (Holland et al., 1998).

However, approximately half of the sources identified in the IRAS faint source catalogue have excess at 25m only (Mannings and Barlow, 1998). The location suggested by this 25m peak puts the dust at an intermediate positions between our asteroid and Kuiper belts, and so directly in the area in which we would expect the formation of gas giants, and therefore no dust!

Possible explanations for this emission are;

• Dust trapped in resonance with a giant planet

• Planetary system is in an intermediary, transitional phase, and so may later develop into a classically cool disk.

• The collisional destruction of asteroids or sublimation of comets within a planetary system.

• These belts are actually the Kuiper belts of failed planetary systems; if so, what prevented their extension beyond ~10AU?

To help determine the origin of these systems, we must first confirm that these excesses are both real and centred on the star; then find their temperatures and so estimate the location of the dust in these systems, using multi-wavelength infra-red photometry.

The stars chosen were those identified by other authors as having 25m excess, together with a new sample taken from the IRAS database (Wyatt et al, in prep).

Techniques

We took observations of our candidate stars using the TIMMI 2 instrument on the 3.6m telescope at La Silla, ESO. The field of view on this instrument allows us to determine whether any excess detected is truly centred on the star. This is important as it has been shown in some cases that this emission is in fact from a background object caught in the IRAS beam, (Sheret et al. 2004, Lisse et al. 2002).

Observations were taken at M, to confirm the photospheric emission of the star and its location, and at longer wavelengths as conditions permitted. Where possible, several wavelengths were examined in order to better constrain the dust temperature.

Excess emission above that expected from the star is plotted separately on the star’s SED, then a modified blackbody curve is fitted to this to constrain the dust temperature, and thus determine its likely radial location.

Figure 1: A example of a TIMMI2 image. The four images resulting for the chop and nod cycles were combined to create our final images.

Results

• A total of eleven of our candidate stars have so far been confirmed as having a mid infra-red excesses.

• 3 sources were found to have a late-type companion at M and N.

• 1 source was found to have a nearby background giant.

We fit the confirmed excess assuming blackbody temperatures for the dust, examples of which are shown in figures 2 and 3.

The images of corvi were also interesting.

Figure 2: The SED of HD80950. The excess emission peaks at around 25m as predicted. The disk emission fit is shown with a dotted line. The temperature of the modified blackbody fit is 180K.

Figure 3: SED of HD109085, corvi. The emission indicates several temperatures of dust.

Corvi

These tentative 3 detections may be confirmed by follow-up 8m work.

Figure 4: The final image from the four combined TIMMI images is magnified and re-scaled. The circled regions indicate apertures placed over these regions of excess flux giving detections at the 3level. The asterisks mark the locations of sub-mm peaks found for this star shown in the third image (Wyatt et al. 2005). Are these peaks related?

If these mid-IR peaks are confirmed, then this system has warm dust much further out form the star then would be expected, making this system different to any previously seen. Imaging on an 8m telescope is essential to our understanding of this system.

Follow-up work

Seven of the eleven stars having confirmed 25m excess are predicted from their blackbody temperature fits to be extended at >0.4 arcseconds. As such we plan to follow-up this study with observations on the Gemini South 8m telescope. Such observations should be able to resolve these disks, verifying their size and determining their inclination. Additionally, any substructure such as has been detected in Kuiper belt-like systems (e.g. Wyatt et al. 1999, Greaves et al., 1998) may be resolvable and indicative of unseen planets in the system.

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• Sheret I., et al., 2004, MNRAS, 348, 1282

• Wyatt M.C., et al., 1999, ApJ, 527, 1321

• Wyatt M. C., 2005, ApJ, 620, 492