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Thread: Does the white dwarf "van Maanen 2" have a planetary system?

  1. #1
    Join Date
    Sep 2004
    South Carolina

    Exclamation Does the white dwarf "van Maanen 2" have a planetary system?

    Surely, one of the most confusing questions in exoplanetary astronomy is, "Does the white dwarf van Maanen 2 have planets, or not?" This question has popped up repeatedly since the early 1970s. Instead of the issue being definitively ruled out, the answer always seems to be, "It might have planets, but if so they are not the planets you think are there."

    Here is a brief yet amusing review of the literature, so far as can be determined.

    An Astrometric Study of Van Maanen's Star
    P. van de Kamp (1971)
    "The residuals from the least squares solution indicate a possible perturbation with a period of some two decades and an amplitude too small to have a sensible effect on the measured acceleration.";filetype=.pdf
    Astrometric determination of the gravitational redshift of van Maanen 2 (EG 5).
    G. Gatewood et al. (1974)
    "Van de Kamp (1971) reported some evidence of a perturbation in £. We too note that the standard error of a single observation of van Maanen 2 was 1.4 times larger in the £ coordinate that in the tj coordinate (the coordinate most involved in the determination of the astrometric radial velocity). In this series of papers, as well as for other stars in this study, the ratio of the standard error of a single observation in £ to that in 77 has generally been 1.0 (Gatewood and Eichhorn 1973; Breakiron and Gatewood 1974). The yearly mean residuals listed in Table IV were plotted against a number of different periods without much success; if orbital motion exists we have failed to find the period.";filetype=.pdf
    Astrometric study of the Sproul plate series on van Maanen's star, including gravitational redshift.
    J.L. Hershey (1978)
    "No clear evidence of short- or long-term orbital periodicity was found.";filetype=.pdf
    Brief search for low-mass objects
    C. Krishna Kumar (1985)
    "Observations of the nearby white dwarfs EG 5 (van Maanen 2) and EG 290 obtained in the J, H, K, L, L-prime, and M bands using an RC1 photometer at the Cassegrain focus of the 3.3-m IR telescope at Mauna Kea on Dec. 2, 1983 are reported. The L-band fluxes are analyzed to search for low-mass companion objects, but none is found. Upper limits of Te = 850 K and L = 5 x 10 to the -6th solar luminosity are established for the putative companions of both dwarfs, corresponding to a mass of 0.027 solar mass or less for an age of 3 Gyr.... The L (3.4 ) band fluxes of two nearby white dwarfs, van Maanen 2 and EG 290, were measured to look for low-mass companions. None were detected.... A companion originally between one and a few AU and a final mass of 0.03 M© would have been detected. In summary, the white dwarf stars EG 5 and EG 290 do not have companions as small as Jupiter and as cool as 900 surface temperature."
    Low-mass Companions to van Maanen 2 and Other Nearby Stars
    V. Makarov (2003)
    "Most of the presently known low-mass companions and planets in extrasolar systems have been discovered via a periodic variation of radial velocity of the primary star. The astrometric method, although currently less accurate, is an alternative and independent way to look for brown dwarf and planetary companions. It is based on the reflex stellar motion caused by the orbital motion in the system. The astrometric method may provide important information on the physical size and inclination of the orbit, and, subsequently, a dynamical estimation of the total and secandary masses. It works fine for objects that may be difficult for the radial velocity search, e.g., very hot primary stars or almost face-on orbits. Using the Hipparcos Intermediate Astrometry Data, we are conducting a systematic screening of nearby (distances less than 20-30 pc) stars, paying special attention to astrometric binaries with considerable accelerations or discrepant long-term and short-term proper motions. Several new systems have been discovered with probable brown dwarf or giant planet companions, including van Maanen 2 (GJ 35), HD 219571, GJ 914A (85 Peg A), GJ 533, GJ 9616 and GJ 9387. Preliminary astrometric solutions are obtained, statistical confidence levels are computed, and secondary masses are estimated. The companion to van Maanen 2 has a mass of about 0.08 Msun, and may be the nearest boundary object between the classes of brown dwarfs and super-giant planets. It orbits the nearest cool white dwarf 3.67 Byr of age, at a distance of just 4.4 pc from the Sun. The estimated period is 1.57 yr, and the predicted semi-amplitude of radial velocity is 0.48 km/s. The maximum separation between the primary white dwarf and the secondary substellar object is approximately 0.3 arcsec, which leaves the possibility of direct imaging of the latter with existing facilities."

    Last edited by Roger E. Moore; 2017-Feb-25 at 02:35 AM. Reason: added missing link
    Do good work. —Virgil Ivan "Gus" Grissom

  2. #2
    Join Date
    Sep 2004
    South Carolina
    The “DODO” survey I: limits on ultra-cool substellar and planetary-mass companions to van Maanen’s star (vMa 2)
    M. R. Burleigh et al. (2008)
    "We report limits in the planetary-mass regime for companions around the nearest single white dwarf to the Sun, van Maanen's star (vMa 2), from deep J-band imaging with Gemini North and Spitzer IRAC mid-IR photometry. We find no resolved common proper motion companions to vMa 2 at separations from 3" - 45", at a limiting magnitude of J~23. Assuming a total age for the system of 4.1 +/-1 Gyr, and utilising the latest evolutionary models for substellar objects, this limit is equivalent to companion masses >7 +/-1 Mjup (T~300K). Taking into account the likely orbital evolution of very low mass companions in the post-main sequence phase, these J-band observations effectively survey orbits around the white dwarf progenitor from 3 - 50AU. There is no flux excess detected in any of the complimentary Spitzer IRAC mid-IR filters. We fit a DZ white dwarf model atmosphere to the optical BVRI, 2MASS JHK and IRAC photometry. The best solution gives T=6030 +/- 240K, log g=8.10 +/-0.04 and, hence, M= 0.633 +/-0.022Msun. We then place a 3sigma upper limit of 10 +/-2 Mjup on the mass of any unresolved companion in the 4.5 micron band."
    Spitzer IRAC Observations of White Dwarfs. II. Massive Planetary and Cold Brown Dwarf Companions to Young and Old Degenerates
    J. Farihi et al. (2008)
    "This paper presents a sensitive and comprehensive IRAC 3 − 8 µm photometric survey of white dwarfs for companions in the planetary mass regime with temperatures cooler than the known T dwarfs. The search focuses on descendents of intermediate mass stars with M & 3 M⊙ whose inner, few hundred AU regions cannot be probed effectively for massive planets and brown dwarfs by any alternative existing method. Furthermore, examination for mid-infrared excess explores an extensive range of orbital semimajor axes, including the intermediate 5 − 50 AU range poorly covered and incompletely accessible by other techniques at main sequence or evolved stars. Three samples of white dwarfs are chosen which together represent relatively young as well as older populations of stars: 9 open cluster white dwarfs, 22 high mass field white dwarfs, and 17 metal-rich field white dwarfs. In particular, these targets include: 7 Hyads and 4 field white dwarfs of similar age; 1 Pleiad and 19 field white dwarfs of similar age; van Maanen 2 and 16 similarly metal-rich white dwarfs with ages between 1 and 7 Gyr. No substellar companion candidates were identified at any star. By demanding a 15% minimum photometric excess at 4.5 µm to indicate a companion detection, upper limits in the planetary mass regime are established at 34 of the sample white dwarfs, 20 of which have limits below 10 MJ according to substellar cooling models. Specifically, limits below the minimum mass for deuterium burning are established at all Pleiades and Hyades white dwarfs, as well as similarly young field white dwarfs, half a dozen of which receive limits at or below 5 MJ. Two IRAC epochs at vMa 2 rule out T >~ 200 K proper motion companions within 1200 AU.... The lack of 4.5 µm excess at all white dwarf targets, especially when combined with similar IRAC searches (Mullally et al. 2007), confirms that L and T-type brown dwarf companions are rare (f < 0.6%) within a few hundred AU, down to masses near the deuterium burning limit. These results suggest the possibility that the lowest mass companions, and especially planets, orbiting intermediate mass stars may be altered or destroyed prior to or during the post-main sequence, or are (more likely) too cold to directly detect with current facilities."
    "0046+051 (vMa 2) At 4.4 pc, this degenerate represents a unique and advantageous hunting ground for planets and planetary system remnants. This cool, helium atmosphere, metal-rich white dwarf has been externally polluted by interstellar or circumstellar matter. Previous ground- and space-based mid-infrared imaging and photometry have ruled out the presence of a substellar companion suggested by Makarov (2004), down to Teff & 500 K and corresponding to around 25 MJ at 5 Gyr (Farihi, Becklin, & Macintosh 2004; Burrows, Sudarsky, & Lunine 2003). The IRAC 4.5 µm photometry of vMa 2 and the models used here rule out the presence of a companion as cool as 400 K, and a mass close to the deuterium burning limit at 4.4 Gyr. Furthermore, the IRAC 4 channel color-color search for resolved substellar companions rules out the presence of any widely bound object as cool as Teff ≈ 550 K within r ≈ 1200 AU of vMa 2. Models predict this should correspond to a mass of 25 MJ at the age of this well studied white dwarf. Deep ground-based J-band imaging observations have ruled out widely-bound planetary mass companions to vMa 2 as small as 7 MJ at orbital separations near 10 − 200 AU (Burleigh et al. 2008). There exist 2 epochs of IRAC 4.5 µm imaging of vMa 2 in the Spitzer archive, separated by 2.1 yr and clearly revealing 6.2 ′′ of proper motion upon blinking the aligned frames. There are no field objects comoving with the white dwarf, ruling out well detected, resolved objects with m4.5µm = 16.7 mag (see Figure 9) or 36 µJy as companions within 1200 AU of vMa 2. At the 4.4 pc distance and 4.4 Gyr age of vMa2, models predict that this very sensitive observational limit corresponds to a mass of 4 MJ and a temperature of just Teff ≈ 200 K; signficantly lower than the known T dwarfs and only 40 K warmer than Jupiter(!)."
    Recognition of the First Observational Evidence of an Extrasolar Planetary System
    B. Zuckerman (2015)
    "In late 2013, Dr. Jay Farihi invited me to give a review talk on the compositions of extrasolar planetesimals and rocky planets at a July 2014 symposium on ”Characterizing Planetary Systems Across the HR Diagram”. He suggested that I include in my review a brief history of the field of heavy element ”pollution” of the photospheres of white dwarfs. While preparing this history I experienced a true ”eureka” moment when I realized that the very first observational evidence for the existence of an extrasolar planetary system came nearly 100 years ago from one of these polluted stars, van Maanen 2. However, proper interpretation of this evidence in terms of the existence of a planetary system in orbit around van Maanen 2 did not come for about another 90 years. The present discussion briefly outlines the story... As time went on more white dwarfs with photospheric metals were discovered, while at the same time it was appreciated that the intense gravitational fields of such stars should cause these heavy elements to sink rapidly out of sight (Schatzman 1945). Thus a source of the metals was required; for decades the favored source was accretion of material from the interstellar medium. But a series of observational papers, a few noteworthy ones are listed in Table 1, accompanied by a series of interpretive papers – early ones include Graham et al 1990, Debes & Sigurdsson 2002, and Jura 2003 – gradually shifted the accretion paradigm from interstellar material to rocky asteroidal debris. If one dates the triumph of the latter model over the former to about 2007, then there was a delay of about 90 years between the discovery of heavy elements in the spectrum of van Maanen 2 and their proper interpretation as originating in a surrounding planetary system."


    You see where I am coming from. Thoughts here?

    My thought: Can it be proven, one way or the other, whether the debris impacting vMa2 is from interstellar space or from its own little planetary/ cometary/ asteroid belt system? Also, the only way I can imagine planetary debris impacting the star is for a perturbing body to pass by and knock stuff out of orbit. vMa2 is pretty old.
    Do good work. —Virgil Ivan "Gus" Grissom

  3. #3
    Join Date
    Sep 2004
    South Carolina
    Plus a number of articles arguing, persuasively, that DZ white dwarfs are eating planets or planetoids or planetesimals whatever. Their own, not ones drifting through space.
    Rocky planetesimals as the origin of metals in DZ stars
    J. Farihi et al., 2010
    The calcium and hydrogen abundances, Galactic positions and kinematics of 146 DZ white dwarfs from the Sloan Digital Sky Survey are analysed to constrain the possible origin of their externally polluted atmospheres. There are no correlations found between their accreted calcium abundances and spatial-kinematical distributions relative to interstellar material. Furthermore, two thirds of the stars are currently located above the Galactic gas and dust layer, and their kinematics indicate multi-Myr residences in this region where interstellar material is virtually absent. Where detected, the hydrogen abundances for 37 DZA stars show little or no correlation with accreted calcium or spatial-kinematical distributions, though there is a general trend with cooling age. It is found that Eddington-type accretion of interstellar hydrogen can reproduce the observed hydrogen abundances, yet simultaneously fails to account for calcium. The calcium-to-hydrogen ratios for the DZA stars are dominated by supersolar values, as are the lower limits for the remaining 109 DZ stars. All together, these polluted white dwarfs currently contain 1020+/-2g of calcium in their convective envelopes, commensurate with the masses of calcium inferred for large asteroids. A census of current Teff <~ 12000K, helium-rich stars from the Sloan Digital Sky Survey suggests the DZ and DC white dwarfs belong to the same stellar population, with similar basic atmospheric compositions, effective temperatures, spatial distributions and Galactic space velocities. Based on this result, pollution by the interstellar medium cannot simultaneously account for both the polluted and non-polluted subpopulations. Rather, it is probable that these white dwarfs are contaminated by circumstellar matter; the rocky remains of terrestrial planetary systems. In this picture, two predictions emerge. First, at least 3.5 per cent of all white dwarfs harbour the remnants of terrestrial planetary systems; this is a concrete lower limit and the true fraction is almost certainly, and perhaps significantly, higher. Therefore, one can infer that at least 3.5 per cent of main-sequence A- and F-type stars build terrestrial planets. Secondly, the DZA stars are externally polluted by both metals and hydrogen, and hence constrain the frequency and mass of water rich, extrasolar planetesimals.
    The White Dwarfs within 25 Parsecs of the Sun: Kinematics and Spectroscopic Subtypes
    Edward M. Sion et al., 2014
    We present the fractional distribution of spectroscopic subtypes, range and distribution of surface temperatures, and kinematical properties of the white dwarfs within 25pc of the sun. There is no convincing evidence of halo white dwarfs in the total 25 pc sample of 224 white dwarfs. There is also little to suggest the presence of genuine thick disk subcomponent members within 25 parsecs. It appears that the entire 25 pc sample likely belong to the thin disk. We also find no significant kinematic differences with respect to spectroscopic subtypes. The total DA to non-DA ratio of the 25 pc sample is 1.8, a manifestation of deepening envelope convection which transforms DA stars with sufficiently thin H surface layers into non-DAs. We compare this ratio with the results of other studies. We find that at least 11% of the white dwarfs within 25 parsecs of the sun (the DAZ and DZ stars) have photospheric metals that likely originate from accretion of circumstellar material (debris disks) around them. If this interpretation is correct, then it suggests the possibility that a similar percentage have planets, asteroid-like bodies or debris disks orbiting them. Our volume-limited sample reveals a pileup of DC white dwarfs at the well-known cutoff in DQ white dwarfs at Tef about 6000K. Mindful of small number statistics, we speculate on its possible evolutionary significance. We find that the incidence of magnetic white dwarfs in the 25 pc sample is at least 8%, in our volume-limited sample, dominated by cool white dwarfs. We derive approximate formation rates of DB and DQ degenerates and present a preliminary test of the evolutionary scenario that all cooling DB stars become DQ white dwarfs via helium convective dredge-up with the diffusion tail of carbon extending upward from their cores.
    The incidence of magnetic fields in cool DZ white dwarfs
    M.A. Hollands et al., 2015
    Little is known about the incidence of magnetic fields among the coolest white dwarfs. Their spectra usually do not exhibit any absorption lines as the bound-bound opacities of hydrogen and helium are vanishingly small. Probing these stars for the presence of magnetic fields is therefore extremely challenging. However, external pollution of a cool white dwarf by, e.g., planetary debris, leads to the appearance of metal lines in its spectral energy distribution. These lines provide a unique tool to identify and measure magnetism in the coolest and oldest white dwarfs in the Galaxy. We report the identification of 7 strongly metal polluted, cool (T_eff < 8000 K) white dwarfs with magnetic field strengths ranging from 1.9 to 9.6 MG. An analysis of our larger magnitude-limited sample of cool DZ yields a lower limit on the magnetic incidence of 13+/-4 percent, noticeably much higher than among hot DA white dwarfs.
    Cristobal Petrovich, et al., 2017
    The presence of a planetary system can shield a planetesimal disk from the secular gravitational perturbations due to distant outer massive objects (planets or stellar companions). As the host star evolves off the main sequence to become a white dwarf, these planets can be engulfed, triggering secular instabilities and leading to the tidal disruptions of small rocky bodies. These disrupted bodies can feed the white dwarfs with rocky material and possibly explain the high-metallicity material in their atmospheres. We illustrate how this mechanism can operate when the gravitational perturbations are due to the Kozai-Lidov mechanism from a stellar binary companion. We show that this mechanism can explain the observed levels of accretion if: (1) the planetary engulfment happens fast compared to the secular timescale, which is generally the case for wide binaries (>100 AU) and planetary engulfment during the Asymptotic Giant Branch; (2) the planetesimal disk has a total mass of ∼10−4−10−2M⊕. We show that this new mechanism can provide a steady supply of material throughout the entire life of the white dwarfs for all cooling ages and can account for a large fraction (up to nearly half) of the observed polluted WDs.

    So, van Maanen 2 has a planetary system, and it is eating it.
    Do good work. —Virgil Ivan "Gus" Grissom

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