We present preliminary limits on the presence of planets around white dwarf
stars using the IRAC photometer on the Spitzer space telescope. Planets emit
strongly in the mid-infrared which allows their presence to be detected as an
excess at these wavelengths. We place limits of $5 M_J$ for 8 stars assuming
ages of $1 Gyr$, and $10 M_J$ for 23 stars.We describe our survey, present our
results and comment on approaches to improve our methodology.Comment: 4 pages, 3 figures, to appear in Proceedings of 15th European White
  Dwarf Worksho

Mullally, F.

von Hippel, Ted

Winget, D. E.

English

arXiv

We present preliminary limits on the presence of planets around white dwarf stars using the IRAC photometer on the Spitzer space telescope. Planets emit strongly in the mid-infrared which allows their presence to be detected as an excess at these wavelengths. We place limits of 5 MJ for 8 stars assuming ages of 1 Gyr, and 10 MJ for 23 stars.We describe our survey, present our results and comment on approaches to improve our methodology

Embry-Riddle Aeronautical University

Publications 10-28-2006 Spitzer White Dwarf Planet Limits F. Mullally University of Texas Ted von Hippel University of Texas, vonhippt@erau.edu D. E. Winget University of Texas Follow this and additional works at: https://commons.erau.edu/publication  Part of the Stars, Interstellar Medium and the Galaxy Commons Scholarly Commons Citation Mullally, F., von Hippel, T., & Winget, D. E. (2006). Spitzer White Dwarf Planet Limits. ASP Conference Series, 372(). Retrieved from https://commons.erau.edu/publication/1261 This Article is brought to you for free and open access by Scholarly Commons. It has been accepted for inclusion in Publications by an authorized administrator of Scholarly Commons. For more information, please contact commons@erau.edu. **FULL TITLE**ASP Conference Series, Vol. **VOLUME**, **YEAR OF PUBLICATION****NAMES OF EDITORS**Spitzer White Dwarf Planet LimitsF. Mullally, Ted von Hippel, D. E. WingetDepartment of Astronomy, 1 University Station, C1400, Austin, TX78712, USA (fergal@astro.as.utexas.edu)Abstract. We present preliminary limits on the presence of planets aroundwhite dwarf stars using the IRAC photometer on the Spitzer space telescope.Planets emit strongly in the mid-infrared which allows their presence to bedetected as an excess at these wavelengths. We place limits of 5 MJ for 8stars assuming ages of 1 Gyr, and 10 MJ for 23 stars.We describe our survey,present our results and comment on approaches to improve our methodology.1. IntroductionDirect detection of planets around main sequence stars is difficult. Proposedspace missions such as Darwin or Terrestrial Planet Finder will require contrastratios ∼ 10−10 to detect terrestrial mass planets. As discussed in other contri-butions to these proceedings, white dwarf stars (WDs) offer a unique advantagein this regard. The luminosity of a 12 kK WD is orders of magnitude less thana progenitor A star. In the mid-infrared the flux from a large (∼ 5 − 10MJ )planet can rival that of the white dwarf. In Figure 1 we plot a spectrum of a12 kK DA white dwarf from Finley et al. (1997) from 1 to 10µm scaled to adistance of 10 pc. We also show the additional flux contribution of an unre-solved 5 and 10 MJ planetary companion based on models by Burrows et al.(2003). The excess around 4 microns is clearly visible. This excess lies betweenabsorption bands of methane and water. The dashed lines show filter tracingsfor Channels 2 and 4 (referred to as the 4.5 and 8.0µm bands) for the IRAC pho-tometer (Fazio et al. 2004) on the Spitzer space telescope (Werner et al. 2004).By observing an excess at 4.5 compared to other wavelengths we can detect thepresence of a planet2. ObservationsWe obtained infrared photometry of 124 bright (K < 15.5) WDs with temper-atures from 5 − 170 kK at 4.5 and 8µm. Our observations are discussed fullyin Mullally (2006, submitted). Briefly we observed each star simultaneously inboth channels 2 and 4 for a total of 150 seconds. Our flux limit at 8µm wassomewhat less than 0.1mJy. Each star was processed with version S11.0.4 of theSpitzer pipeline to produce basic calibrated data frames (or bcds). We extractedphotometry from each bcd frame using the aper routine in IDL and compared itto visual photometry from McCook & Sion (1999) and near infrared photometryfrom 2MASS.12 F. Mullally et al.Figure 1. Model of a 12 kK white dwarf photosphere scaled to a distanceof 10 pc with the contribution 5 and 10MJ planet included (dark and lightgrey respectively). The IRAC photometer sensitivities for Channels 2 and 4are also shown as dotted lines.3. Models and modelingFor this work we focused solely on the 77 DA stars with temperatures between7 and 60 kK (the range of our model grid) in our sample. We created a modelspectrum of each WD based on its temperature (and assuming log g = 8.0) bylinearly interpolating over a grid of models from Finley et al. (1997). To thisspectrum we added the flux of a planet model from Burrows et al. (2003) for arange of masses from 1 to 25MJ and ages of 0.3, 1.0 and 3 Gyr. Performingsynthetic photometry of these combined models yielded the model flux at V, 4.5and 8.0.We then compared the ratio of these fluxes (analogous to colour), to thatobserved from the DAs in our sample. Figure 2 shows this flux ratio for anexample star in our study. As the mass of the planet increases the flux at 4.5,and to a lesser extent 8.0µm, increases as compared to V. Looking only at 4.5,we determine a mass limit as the mass of planet that would produce an excessequal to the the position of the right hand 1σ error bar. For the figure shownabove, our limit is < 5 MJ .Our results are shown in Figure 3. The lower axis is the 1σ upper mass limit,the left axis is the number of systems for which this limit was achieved. Becausewe do not yet have age information about these systems, we calculated limitsfor 3 reasonable planet ages. The squares show the number of planets for whicha given upper mass limit was achieved for 1 Gyr old planets. There are 8 starsin our survey for which no planets with masses above 5 MJ and ages of 1 Gyrwere detected, and 23 stars with no planets detected above 10 MJ . The circlesand diamonds show limits for younger (0.3 Gyr) and older (3 Gyr) planets. Fora 12 kK WD of an A star progenitor, the age of the system (progenitor lifetimeplus WD cooling age) is ≈ 1.5 Gyr.For a number of stars, the measured flux ratio lies to blue of the syntheticphotometry of an unaccompanied WD. This corresponds to an observed fluxdeficit at 4.5 relative to V. This causes the 1σ limit on planetary companions toSpitzer White Dwarf Planet Limits 3Figure 2. Flux ratios for a typical star in our survey. The presence of anunresolved low mass companion around the white dwarf increases the observedflux at 4.5 and 8.0µm compared to the optical photometry. The 1σ limit onplanets around this star is better than 15MJ .be unreasonably low, and these stars were excluded from our result. We placedlimits on a total of 39 stars.4. ImprovementsAs discussed elsewhere in this volume, comparing Spitzer fluxes to V band mea-surements can lead to false deficit measurements, which may explain the anoma-lously low limits we encountered. A better approach would be to measure theexcess over a model normalized to all available visible and near-IR photometry,or to directly fit the observed spectral energy distribution to a grid of whitedwarf + planet models, which we intend to do in the near future. Another con-cern is the recent paper of Patten et al. (2006) who found that T dwarves showa smaller than predicted flux in IRAC’s Channel 2. They attribute this to largerthan expected CO absorption, and may reduce the sensitivity of our survey.5. ConclusionWith their low luminosity relative to their progenitors, WDs over a uniqueopportunity to directly detect the flux from a companion planet. In this first-look survey with the Spitzer Space Telescope, we surveyed a large number (124)white dwarf stars, but found no convincing evidence of a planet. In this workwe make a first cut at placing limits on planets using this data. We find thatfor planets between the ages of 0.3 and 1 Gyr we can rule out the presence ofplanets with masses larger than between 8 and 16 MJ for 30 systems.Acknowledgments. This work is based on observations made with theSpitzer Space Telescope, which is operated by the Jet Propulsion Laboratory,4 F. Mullally et al.California Institute of Technology under NASA contract 1407. Support for thiswork was provided by NASA through award project NBR 1269551 issued byJPL/Caltech to the University of Texas. This work is performed in part undercontract with the Jet Propulsion Laboratory (JPL) funded by NASA throughthe Michelson Fellowship Program. JPL is managed for NASA by the CaliforniaInstitute of Technology. One of us (FM) wishes to thank the Royal AstronomicalSociety for a generous grant. Support of the Royal Astronomical Society as wellof the American Astronomical Society and the National Science Foundation inthe form of an International Travel Grant, which enabled one of us (FM) toattend this conference.ReferencesBurrows, A., Sudarsky, D., & Lunine, J. I. 2003, ApJ, 596, 587Fazio, G. G., Hora, J. L., Allen, L. E., et al. 2004, ApJS, 154, 10Finley, D. S., Koester, D., & Basri, G. 1997, ApJ, 488, 375McCook, G. P. & Sion, E. M. 1999, ApJS, 121, 1Patten, B. M., Stauffer, J. R., Burrows, A., et al. 2006, astroph/0606432Werner, M. W., Roellig, T. L., Low, F. J., et al. 2004, ApJS, 154, 1Spitzer White Dwarf Planet Limits 5Figure 3. Cumulative distribution of planet limits from this work for threereasonable planet ages. The circles are for planet models of 0.3 Gyr, squaresare for 1.0 Gyr and diamonds for 3.0 Gyr. The vertical dashed line indicatesthe boundary mass between planets and BDs as defined by the deuteriumburning limit. The presence of planets with masses greater than 13MJ andages less than 1 Gyr has been ruled out around 24 stars in our sample, whileplanets with masses above 5 MJ have been ruled out for 8 objects. Youngerplanets are brighter and easier to detect, so have more stringent limits. Plan-ets with masses more than 5 Gyr and ages of 0.3Gyr are ruled out for 24 stars,while the limit is as low as 2 MJ for 8 stars.

Spitzer White Dwarf Planet Limits

https://commons.erau.edu/cgi/viewcontent.cgi?article=2267&amp;context=publication

Spitzer White Dwarf Planet Limits

Abstract

Similar works

Full text

Available Versions

Embry-Riddle Aeronautical University