WDS TPF-FS program

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Duplicity, Binarity, and Masses for TPF

The Washington Double Star Catalog

Terrestrial Planet Finder - Foundation Science Program

Proposal Abstract : Double stars have proven to be both a blessing and a curse for astronomers since their discovery over two centuries ago. They remain the only reliable source of masses, the most fundamental parameter defining stars. On the other hand, their sobriquet "vermin of the sky" is well-earned, due to the complications they present to both observers and theoreticians. These range from non-linear proper motions to stray light in detectors to confusion in pointing of instruments (e.g., star trackers) due to non-symmetric point spread functions to angular momentum conservation in multiple stars which results in binaries closer than allowed by evolution by two single stars. This proposal is an effort to address both their positive and negative aspects, through speckle interferometric observations of a selection of single and double stars of interest to the Terrestrial Planet Finder mission.

While there are over 100,000 known doubles, this proposal will concentrate on the ~2400 systems where useful information can be obtained with only a single observation. We propose to examine a selection of potential TPF target stars for possible duplicity effects which could complicate the satellite's mission. The proposed work will also refine current statistics regarding duplicity (chance alignments of nearby point sources) and binarity (actual physical relationships), and improve the precisions and accuracies of stellar masses.

Target List Overview :

TPF Targets : Objects on the current TPF list to be observed fall into two categories:
- Presumably single stars : Even the most well-studied stars previously thought to be single may reveal hitherto-unknown companions when studied using different techniques. Speckle interferometry is sensitive to a portion of the separation regime not easily accessible by spectroscopy or other techniques.
- Known binaries : Well-separated binaries can be quite profitable for examination by TPF, as they can have twice the habitable zone volume as compared with single stars. While they do not fit the criticality issues as enumerated above, their identification as potential TPF pairs makes investigation of them inherently more interesting. In these cases the precision and accuracy afforded by these observations will allow their current relative positions to be known and their future positions predicted with the greatest degree of certainty.
Confirmation Doubles : These are doubles having only a single resolution in the WDS. While duplicity can be assumed based on one observation, past results indicate that taking these at face value can lead to over-estimation of the duplicity fraction.
Motion Characterization Doubles : These are doubles with two or more observations of sufficient accuracy and precision, separated in time and space, which do not exhibit clear Keplerian motion. A single high-quality resolution of these doubles should allow us to determine if the motion as rectilinear or not, thus establishing the pair as either an optical double star or a bona fide binary star.

Despite the close angular separation, their true nature, physical or optical is unknown. Due to the accuracy and precision of speckle interferometry, this can be determined with one or more resolution. Boxes indicate where the secondary should be assuming the motion is linear in 2005, 2006, 2007 & 2008.

Orbit Binaries : While there are a significant number of binary stars with orbits observable with the 100", not all of these need to be observed. Those that do fall into one of three categories:
- Quadrant Ambiguity Pairs : When the magnitude difference of a pair is small, the inability to definitively distinguish between the two components can result in two orbits consistent with the data: a shorter-period, high-eccentricity solution and a longer-period low-eccentricity orbit. The orbit systems to be observed are ones where the two orbits predict significantly different separations and/or position angles for the components during the observing seasons included in this proposal.

Two possible solutions of a "quadrant ambiguity" system. Both orbits are quite good and show very small residuals, however, over the period 2006.5-2007.5 (2007.0 indicated with a star), these two orbits have very different predictions.

Critical Phase Pairs : Errors in orbital elements are significantly reduced when the orbit is characterized over all phases. Most of these pairs have orbits which predict the companion to be in a portion of the orbit currently lacking data. Others will be just completing one revolution since their discoveries, so in each case an additional observation would significantly tie down the orbital period.

These speckle binaries are approaching one complete revolution. An observation soon will tie down the orbital period quite well, improving mass errors in orbit analysis. The shaded wedge here indicates the period 2005-2008 with 2007 indicated with a star.

Orbital Runoff Pairs : For these pairs, the published orbit matched historical data well, but newer data are beginning to indicate a systematic trend in the residuals which would indicate the orbit is in error. The objective here is to verify the trend and then determine new orbital elements.

While the orbits historically fit the data quite well, there is evidence of orbital runoff in these systems. The 2005-2008 shaded wedge is based on the obviously incorrect orbit and thus, may be off quite a bit.

Observing Site and List

Observations will be made on the Mt Wilson 100" telescope using the USNO speckle camera. The USNO camera has been used on a variety of telescopes (CTIO 4-m, KPNO 4-m, McDonald 2.1-m, NOFS 1.55-m) observing doubles as faint as 16th magnitude and as close as the Rayleigh resolution limit of the relevant telescope. Software was written at the USNO and is maintained in house. Previously, the CHARA speckle camera was used successfully on the Mt Wilson telescope during 22 observing runs (~100 nights) over a span of five years. The USNO speckle camera is a 2nd generation version of the CHARA camera and utilizing it on the 100" will present no technical difficulties. Calibration will be obtained with a full aperture double slit mask constructed specifically for calibration of speckle data on this telescope.

Mt Wilson Observatory, outside Los Angeles, has been a center for interferometry work from early in its inception to the present day. Although perhaps surprising at first to think of a working astronomical facility so close to a major city, it should be noted that interferometers typically observe only bright stellar targets which are not compromised by light pollution. Lying above the usual inversion layer of the Los Angeles valley, Mt Wilson is part of the first mountain range encountered by the laminar flow of air across the Pacific; the exceptional seeing this often yields more than compensates for any faint-object limitations imposed by the city below.

The observing site and telescope for these observations are the Hooker telescope on Mt Wilson. The site has exceptional seeing characteristics, a critical factor in speckle observing and the observers have significant experience with the 100in telescope.

The list indicates the position of all targets in the proposed program, taking into account the observing latitude and the possible coverage of the Hooker telescope. Objects not observed in this program would be appropriate for another telescope and location.

Placement of a full-aperture double-slit mask over the front end of the telescope allows for scale determination to exceptional accuracy, aiding significantly in the kinematic and dynamic disentanglement needed for the targets above. The speckle observing being conducted from the dome floor of the 100" is shown at right.

Intensity transforms of slit mask calibration data taken with the Mt Wilson 100" telescope.

After collecting stellar data through the speckle camera and processing it the images above are generated for the slit-mask. The regularity of the peak spacing follows the precepts of Dr. Young as set down in his classic experiment, which allows for the precision and accuracy these analyses require.

Directed vector autocorrelation of a known binary (HDS2588) and a single star (HIP 88972).

In the examples above, the known binary (a "confirmation double" in the TPF target list above) was first resolved by the Hipparcos mission in the early 1990s and remained unconfirmed until this observation. The difference in peak heights to the left and right of the zeroth order correlation is due to the magnitude difference of the pair. The single star (see above list) shows no hint of a companion. If it has one it is closer than the Rayleigh V band resolution limit of the 100" telescope (0.055") and/or has a magnitude difference greater than three.

Surface and "3d" plot of two orbit stars from the target list above.

In the top two plots are the binary FIN 332Bb while the bottom two plots are FIN 332Aa. While Bb is clearly resolved, Aa is unresolved. Analysis of this complex multiple system, dubbed Tweedledee and Tweedledum by the great visual interferometrist William S. Finsen, is ongoing and will await final calibration.

Results

The list indicates targets observed on the first three speckle runs.

The July 2006 run was quite successful. Six out of eight nights were effective and 526 observations were obtained. Of these, 241 produced double-star measures of high accuracy and precision. Some 132 allegedly single stars were examined for duplicity under excellent conditions and can be characterized as single within the detection limits set by the telescope and technique.
The April 2007 run also had marked success. Four out of six nights were effective, and 446 observations were obtained. Of these, 193 produced accurate double-star measures and an additional 74 allegedly single stars were confirmed as single within our resolution and magnitude limits.
The October 2007 run unfortunately coincided with Santa Anna induced fires throughout much of Southern California; the resulting smoke and ash prevented us from opening the telescope mirror for many otherwise-clear nights. Despite this, three out of nine nights were effective, and 569 observations were obtained, 287 of which produced measures of high quality and 240 of which can be characterized as single within the detection limits set by our equipment.

Plans for 2008

The list indicates targets not observed on the first three speckle runs.

Approximately 57% of the Speckle/TPF Mt Wilson list remains to be observed. Observing will be scheduled to complete this interfacing with Mt Wilson administrative staff to ensure observing is selected that allows remaining objects to be observed and has the greatest potential for favorable observing conditions.