The CRIRES Search for Planets Around the Lowest-Mass Stars. II. The Proposed Giant Planet Orbiting VB10 Does Not Exist

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by  Jacob L. Bean

Accepted for publicationin ApJ Letters
PreprinttypesetusingLATEXstyleemulateapjv.03/07/07
THE CRIRES SEARCH FOR PLANETS AROUND THE LOWEST-MASS STARS.
II. THE PROPOSEDGIANT PLANET ORBITING VB10 DOES NOT EXIST
Jacob L. Bean2,7, Andreas Seifahrt2,3, Henrik Hartman4, Hampus Nilsson4, Ansgar Reiners2,8, Stefan Dreizler2,
Todd J. Henry5, & Gu¨nter Wiedemann6,
Accepted for publication in ApJ Letters
ABSTRACT
0 We present high-precision relative radial velocities of the very low-mass star VB10 that were ob-
1 tained over a time span of 0.61yr as part of an ongoing search for planets around stars at the end
0 of the main sequence. The radialvelocities were measured from high-resolutionnear-infraredspectra
2 obtained using the CRIRES instrument on the VLT with an ammonia gas cell. The typical internal
precision of the measurements is 10ms−1. These data do not exhibit significant variability and are
n
a essentially constant at a level consistent with the measurement uncertainties. Therefore, we do not
J detect the radialvelocityvariationsof VB10expected due to the presence ofanorbiting giantplanet
6 similar to that recently proposed by Pravdo and Shaklan based on apparent astrometric perturba-
2 tions. In addition, we do not confirm the ∼1kms−1 radial velocity variability of the star tentatively
detected by Zapatero Osorio and colleagues with lower precision measurements. Our measurements

] rule out planets with Mp>3MJup and the orbital period and inclination suggested by Pravdo and
P
Shaklanat better than 5σ confidence. We conclude that the planet detection claimedby Pravdoand
E
Shaklanisspuriousonthebasisofthisresult. Althoughtheoutcomeofthisworkisanon-detection,it
. illustratesthepotentialofusingammoniacellradialvelocitiestodetectplanetsaroundverylow-mass
h
p stars.
- Subject headings: planetary systems — stars: individual (GJ752B)
o
r
st 1. INTRODUCTION M⋆<0.2M⊙) using high-precision near-infrared (NIR)
a radial velocity measurements (Bean et al. 2009). The
Very low-mass stars are an interesting sample of po-
[
advantages of the NIR for radial velocity measurements
tential planet hosts. They are useful for probing the
2 correlation between gas giant planet frequency and stel- arethatitispossibletoobtainhigh-resolutionandhigh-
v lar mass (Endl et al. 2006; Johnson et al. 2007), are the signal-to-noise spectra of very low-mass stars in these
3 most numerous stars in the Galaxy (Henry et al. 2006), wavelengths with current instruments, and also that the
0 affect of activity induced “jitter” is potentially reduced
andexhibitalargerdynamicalresponsetoorbitingplan-
0 relative to the visible (Reiners et al. 2009). We present
ets and have closer-in habitable zones than higher-mass
0 hereourradialvelocitymeasurementsofVB10obtained
stars(Kasting et al.1993). However,thesearchforplan-
. so far, and discuss them in the light of PS09’s claimed
2 ets around these stars is hindered by their extreme in-
1 trinsicfaintness atvisible wavelengths,andfew sensitive detection. Zapatero Osorio et al. (2009, hereafter Z09)
9 searches for planets around them have been carried out. have presented NIR radial velocities with a typical in-
0 Recently, Pravdo & Shaklan (2009, hereafter PS09) ternal precision of 300ms−1 that were obtained using
: NIRSPEConKeck(McLean et al.1998). Thesedataex-
v have claimed the first detection of a planet around hibit variability on the level of ∼1kms−1, which would
i a nearby, very low-mass star based on apparent peri-
X be consistent with the existence of a giant planet. Our
odic astrometric perturbations. The star is VB10 and
radial velocity measurements, which are 30 times more
r the planet candidate is a giant planet with a mass
a precisethanthoseintheZ09study,alsoallowustomake
M =6.4M and orbital period P=0.744yr. This
p Jup
a direct comparison to their results.
could also be the first astrometric discovery of an ex-
oplanet.
We have been observing VB10 as part of an ongo- 2. RADIALVELOCITYMEASUREMENTS
ing search for planets around the lowest-mass stars (i.e. We obtained spectroscopic observations of VB10 for
radial velocity measurements using the high-resolution
Electronicaddress: [email protected]
NIR spectrograph CRIRES (K¨aufl et al. 2004), which is
1Based on observations made with ESO Telescopes at the
fed by the UT1 telescope of the Very Large Telescope
ParanalObservatoriesunderprogramID182.C-0748
2Institutfu¨rAstrophysik,Georg-August-Universit¨at,Friedrich- (VLT) facility. As with all the measurements obtained
Hund-Platz1,37077G¨ottingen, Germany as part of our CRIRES planet search, a glass cell filled
3Department of Physics, University of California, One Shields
withammoniagaswasplacedinfrontofthespectrograph
Avenue,Davis,CA95616,USA
4LundObservatory,LundUniversity,Box43,22100Lund,Swe- entrance to enable high-precision calibration of the in-
den strument response during analysis of the data. We used
5DepartmentofPhysicsandAstronomy,GeorgiaStateUniver- a 0′.′4 slit for the VB10 observations, which corresponds
sity,Atlanta, GA,30302,USA
6Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, to a nominal resolving power R ≡ λ/∆λ ≈ 50000, and
Germany werecordedspectraintheCObandheadregionoftheK-
7MarieCurieInternational IncomingFellow band(∼2.3µm). Thisisaregioninwhichlate-typestars
8EmmyNoetherFellow exhibit useful lines for radialvelocity measurements and
2 Bean et al.
TABLE 1
RadialVelocities OfVB10
HJD-2450000.0 RV(ms−1)
4919.89386 8.7 ± 7.1
4922.92002 7.5 ± 15.8
4996.79732 -15.4 ± 5.8
4998.78129 3.8 ± 10.4
4999.75673 -12.7 ± 6.6
5000.78619 -3.1 ± 7.2
5053.67793 13.3 ± 11.8
5054.66721 19.9 ± 7.5
5141.49203 2.2 ± 12.5
5142.49130 7.6 ± 15.5
5143.48948 11.8 ± 10.0 Fig. 1.— Radial velocities for VB10 measured using CRIRES
5144.50118 -5.1 ± 11.1 at the VLT with an ammonia gas cell. Multiple exposures taken
withinupto2.5hhavebeenbinnedtogiveeachpoint.
marily limited by the obtained SNR in both the spectra
theammoniacellalsoexhibitsusefulcalibrationlines. A
that were analyzed to give the measurements, and also
total of 36.4nm of the 48.2nm of recorded wavelength
the template spectrum utilized for the analysis.
coverage was utilized for the radial velocity measure-
Wenormallycombinetheradialvelocitymeasurements
ments.
fromspectratakenwithinashorttimeframetogiveone
Observations were obtained during 12 nights in four
velocity value. For VB10, we binned all observations
classical style runs between March and November 2009.
obtainedover2.5hrintervals(4–16individualmeasure-
Eachvisitconsistedofasequence(typicallyfour)of300s
ments) to give the final data set used in this study be-
exposures. We reduced the data and measured relative
causethisisashortamountoftimerelativetotheorbital
radial velocities from the extracted spectra using the
period of the putative planet we aimed to detect. The
methodsdescribedbyusinapreviouspaper(Bean et al.
resulting radial velocities are given in Table 1 (with the
2009). Our radial velocity measurement algorithm is an
bestfitoffsetsubtracted-seethenextsection)andplot-
adaptation of the canonical gas cell technique described
ted in Figure 1. The typical uncertainty in the values is
by Butler et al. (1996), and includes simultaneous mod-
10ms−1.
elingofthewavelengthscaleandinstrumentpointspread
function in each spectrum using the ammonia cell lines 3. ANALYSIS
as a fiducial. We havepreviously demonstrated that our
method can deliver long-term precisions of 5ms−1 for 3.1. Comparison of models
late M dwarfs when applied to data of sufficient qual- The radial velocity measurements we have obtained
ity (i.e. signal-to-noise and resolution) obtained with exhibit an rms dispersion of 10.6ms−1 around a con-
CRIRES using the ammonia cell. We concentrated only stant value when binned over 2.5h, which is consistent
on measuring and analyzing relative radial velocities for with our estimated uncertainties. Therefore, we have
this study because our gas cell method enables us to not detected reflex orbital motion induced on VB10 by
reach velocity precisions on an internal scale that are an unseen companion. Therefore, the question becomes
roughly an order of magnitude better than we could ob- whetherornotourdataareinconsistentwiththepredic-
tainforvelocitiesonanabsolutescale. Thehigherpreci- tionoftheexistenceofagiantplanetfromPS09,andthe
sionwe obtainforrelativeasopposedto absoluteveloci- potential detection of radial velocity variability by Z09.
tiesyieldsthebestsensitivityforthedetectionofunseen Our measured radial velocities are plotted along with
companions orbiting VB10. some possible orbit models in Figure 2. The data span
The signal-to-noise ratios (SNR) of the reduced and 82% of the orbital period (0.744yr) suggested by PS09,
extracted 1d spectra of VB10 are ∼100pixel−1. The insteps of∼0.25phaseunits. Giventhatouruncertain-
expected photon-limited radial velocity precisions from ties are roughly two orders of magnitude smaller than
these data are typically 13ms−1 including calibration the semi-amplitude expected for a giant planet with an
uncertainty, and the median of our estimated uncer- orbital period of a few hundred days (&1kms−1), it is
tainties on the radialvelocity measurements is 22ms−1. likely that we would have detected significant changes
Our estimated uncertainties are normally larger than in the star’s radial velocity over the time span the data
the correspondingphoton-limitedprecisions because our were obtained if such a planet exists.
error estimation method also accounts for the imper- As discussed in §2, our measurements, like all other
fectspectrummodelingduringthemeasurementprocess. high-precisionradialvelocity measurements, are relative
Previous observations of radial velocity standard stars to an arbitrary scale. We have determined a best fit
have shown that our adopted measurement uncertain- constant offset for all the orbits shown in this paper to
ties are reasonable (down to the long-term noise floor of account for this. We have fixed the orbital period and
∼5ms−1), and we have found that the determined un- inclination(i=96.9◦)to the values givenbyPS09forall
certainties for our method are usually a factor of 1.5 – the analyses presented in this paper because our aim is
2.5largerthanthephoton-limitedprecisions(Bean et al. to assess their specific claim of a planet detection. The
2009). Thatourestimateduncertaintiesonthevelocities correspondinguncertaintiesintheperiodandinclination
in this case are typically 1.7 times the expected photon- given by PS09 do not have a significant influence on the
limited error is completely consistent with this previous modelsrelativetotheprecisionofourdata. Forexample,
result. Ultimately, our VB10 radial velocities are pri- adjustingtheassumedperiodandinclinationwithintheir
NO GIANT PLANET ORBITING VB10 3
servations. For reference, the best fit time of periastron
is 2455103, and the best fit longitude of periastron is
10◦. However, the models do not fit the data even with
these optimizations. The lower panel in Figure 2 shows
the residuals from the M =6M model. The rms of
p Jup
the residuals is 256ms−1, which is much largerthan the
observed dispersion in the data and the uncertainties in
the individual measurements.
3.2. Limits to a giant planet
We conclude from the comparisons described above
that it is unlikely that our radial velocity measurements
arecompatiblewiththepresenceofagiantplanetaround
VB10withanorbitalperiodof0.744yrandnearlyedge-
on orientation. Therefore, we were motivated to under-
take a more specific and robust determination of what
the data rule out. We did this by mapping the quality
of orbit model fits to the data as a function of the pos-
sible parameters, and comparing the determined values
to the fit quality of a model representing no radial ve-
locity variations (i.e. a flat line). It can be seen from
inspection of Figure 2 that higher eccentricity and lower
mass planets could be more easily hidden in the data.
This led us to map the fit quality as a function of possi-
ble planet mass and orbital eccentricity. We considered
Fig. 2.— Top Measured radial velocities for VB10 phased to planet masses ranging from 0.5 to 15M , and eccen-
Jup
the orbital period given by PS09 (points). The data are binned tricity values ranging from 0.0 to 0.95. We again fixed
over each observing run for clarity and the error bars are smaller
theperiodandinclinationtothosevaluesgivenbyPS09,
than the symbols. The dashed orange line gives the orbit model
suggested by Z09 with the relative offset determined to give the and we marginalized over the unconstrained parameters
best fit. The solid dark blue line gives the best fit to the data at each grid point.
for a model with a 6MJup planet on an orbit with e=0.5. The Figure3showstheobtainedmapofthefitχ2. Forref-
light blue solid lines give the best fit models for 3MJup (smaller erence,theχ2forafitofaflatlinetothedatais23.1with
semi-amplitude) and 9MJup (larger semi-amplitude) planets also
with e=0.5. Bottom Residuals from the best fit 6MJup planet 11 degrees of freedom. This somewhat higher than ex-
model(points). Thegreybargivestherangeofvaluesthatwould pectedχ2suggeststhateitherVB10isexhibitingrealra-
b(±e3co2nmsiss−te1n)t.withthreetimestheobserveddispersioninthedata dialvelocityvariabilityatthe10ms−1level,jitterarising
fromactivityisaffectingthemeasurementsalsoatabout
the10ms−1 level(notethatVB10isknowntobeavari-
1σ uncertainties to obtain the smallest possible velocity able source with some flare activity, Berger et al. 2008),
semi-amplitude can only yield a maximum reduction in wehaveunderestimatedouruncertaintiesbyabout50%,
the semi-amplitude of <3%. In addition to the period or some combination of these effects. At this point it
andinclination,wealsoassumedthesamemassforVB10 is not critical to understand the origin of the additional
that PS09 adopted (M⋆=0.078M⊙) to stay consistent variability in the radial velocity measurements because
with their study. this investigation is focused on ruling out much larger
The orbit model that was found by Z09 to give the variations.
best fit to both their NIRSPEC radialvelocities and the The 5σ limit shown in Figure 3 corresponds to the
PS09 astrometry is shown in Figure 2 for comparison boundary where the fit χ2 increases by more than 25
withourdata. The rmsofthe residualsofourdatafrom fromthe χ2 ofthe flatline fittothe data. Everythingto
thismodelwiththerelativeoffsetoptimizedis870ms−1. thelowerrightofthislineisruledoutathighconfidence.
Therefore, the Z09 model is inconsistent with our mea- We find that for all conceivable eccentricity values (i.e.
surements. e<0.95) we can rule out planets with M >3M and
p Jup
Some additional orbit models for planets with masses the orbital period and inclination suggested by PS09.
of 3, 6, and 9MJup are shown in Figure 2. This range of Thismeansthatwecandefinitivelyruleoutplanetswith
massesroughlycorrespondstothenominalvalueandup- massesinPS09’sgiven1σ confidenceinterval(indicated
per and lower 1σ confidence limits determined by PS09. by the dottedlines in Figure 3). All highermass planets
Thetime andlocationofperiastron,andtheeccentricity are also ruled out. At the lower range of possible planet
of the candidate planet detected by PS09 are not con- massesthedeterminedlimitisdependentontheassumed
strained by their astrometry data. For these example eccentricity. For example, planet masses of 1M and
Jup
models we adopted a fixed eccentricity e=0.5, and op- 0.5M can only be ruled out for e<0.7 and e<0.5
Jup
timized the remaining parameters to give the best fit to respectively.
thedata(timeofperiastron,longitudeofperiastron,and
radial velocity offset). As can be seen in the figure, the 4. DISCUSSION
optimization of the unconstrained parameters leads to WehavepresentedradialvelocitiesforVB10thatwere
the orbital phases when the largest velocity variations obtained in the NIR using a new type of gas cell and
are expected to be outside the times covered by our ob- that have a typical internal precision of 10ms−1. These
4 Bean et al.
(2010). Therefore, we suggest that the planet detection
claimedby PS09is spurious. If further workverifies this
result then it would still be the case that no planet has
yet been discovered via astrometry, and that the cur-
rent state-of-the-art for the astrometric study of exo-
planets remains post priori detection and characteriza-
tionofa few selectplanetsthat werefirstidentifiedwith
the radial velocity method (e.g. Benedict et al. 2002;
McArthur et al. 2004; Bean et al. 2007; Martioli et al.
2010). Wenotethatobtainingastrometricmeasurements
at the level of precision needed to detect exoplanets is
notoriously difficult, especially from the ground as PS09
haveattemptedtodo. Thereisalonghistoryofclaimed
andlaterrefutedastrometricexoplanetdetectionsdating
back to the famous case of the possible planet around
Barnard’s star, which was originally suggested based on
25 years of observations by van de Kamp (1963) but
later refuted by Hershey (1973), Gatewood & Eichhorn
(1973), and others. Furthermore, PS09 were not even
Fig. 3.— Map of the best fit χ2 for a planet with P=0.744yr
and i=96.9◦. At each point in the map the time of periastron, the first to propose the existence of a sub-stellar com-
longitude of periastron, and radial velocity offset were optimized panion to VB10 based on apparent astrometric pertur-
to give the best fit. The 5σ upper limit is given by the dashed bations(Harrington et al.1983). Thefirstcandidatewas
contour. The region to the lower right of this line is excluded by
laterdisprovedbysubsequentastrometricmeasurements
ourdata. Thedottedlinesdelineatethe1σconfidenceintervalon
theplanetmassfromPS09. (Monet et al. 1992; Harrington et al. 1993).
With regards to the possible radial velocity variations
data were obtained in four epochs over a time span of in VB10 seen by Z09, we suggest their analysis could
0.61yr. Our radial velocity measurements do not ex- have been influenced by an important systematic effect.
hibit significant variability and are essentially constant This is a possibility that those authors themselves ac-
at a level consistent with the measurement uncertain- knowledged. OfthefivemeasurementspresentedbyZ09,
ties. Therefore, we see no evidence for a giant planet theonepointintheirdatasetthatisinconsistentwitha
orbiting VB10 similar to that detected via apparent as- constant radial velocity (i.e. no variations possibly due
trometricperturbationsbyPS09. Inaddition, wedonot to orbital motion) is also based on a spectrum that was
confirmthe∼1kms−1variabilitytentativelydetectedby taken with a different slit width than all the other data.
Z09 using NIRSPEC radial velocity measurements that Z09didnotuseadeconvolutiontechniquetoaccountfor
have a typical internal precision of 300ms−1. Our data changesinthespectrographpointspreadfunction,sothe
are also inconsistent with the orbit model presented by velocitymeasuredfromthisspectrumcouldhaveadiffer-
Z09 as giving the best fit to both the PS09 astrometry ent relative zero point compared to the other data. It is
and their radial velocities combined. If we consider our not clear whether the different slit width would cause as
radial velocity data alone, then we can rule out plan- large a discrepancy as they saw (∼1kms−1), but it is a
ets with M >3M in the orbit suggested by PS09. possible candidate for reconciling the difference between
p Jup
Planets with masses down to 1M would need to have their measurements and ours.
Jup
unusually large orbital eccentricities and be phased in Despite the negative result indicated by our data, we
a particular way to not have caused highly significant propose that there is a positive general result from this
variations in our measured radial velocities. work. Namely, that we have demonstrated the power
After we submitted our paper we became aware of NIR radial velocities obtained using CRIRES at the
of another paper discussing the proposed VB10 VLT with our ammonia cell. Late-type M dwarfs are
planet that was submitted at roughly the same time aninterestingpotentialplanethostsample,butthey are
(Anglada-Escude et al. 2010). The authors of that pa- also extremely challenging for the normal planet search
per presented radial velocity measurements for VB10 methods due to their faintness. For example, VB10 is
with a typical precision of ∼200ms−1 and that cov- one of the closest, and therefore brightest, stars with an
eredroughlythe sametime periodasourmeasurements. estimated mass <0.1M⊙, but it still has V =17.5. This
Theirdataalsoindicateanon-detectionofthereflexmo- putsitfarbeyondthereachofhigh-precisionradialveloc-
tion of VB10 expected due to the presence of the giant itymeasurementsinthevisiblewithcurrentinstruments.
planet proposed by PS09. Anglada-Escude et al. (2010) However,lateMdwarfsarereasonablybrightintheNIR
carried out a combined analysis of the PS09 astrometry duetotheirextremeredness,andVB10hasK=8.8. By
and their radial velocities and found that the possible developingatechniqueformeasuringhigh-precisionNIR
orbit models do not give a significantly better fit to the radial velocities, we have opened the door to a sensitive
data than a model assuming no reflex motion. They search for planets around stars at the very end of the
concludedthatthe giantplanetproposedby PS09is not main sequence and even beyond. The method is a valu-
likelytoexistonthebasisoftheseresults,althoughthey ablestand-alonetool,andalsoacomplementtotheongo-
couldn’t completely rule it out with their data. ing astrometric planet searches targeting M dwarfs (e.g.
Ourresultsseemtocompletelyruleoutthepresenceof Pravdo et al. 2004; Henry et al. 2006; Boss et al. 2009;
a giant planet similar to that proposed by PS09, which Forbrich & Berger 2009). We are continuing to obtain
is consistent with the results of Anglada-Escude et al. radialvelocitymeasurementsforVB10,andmany other
NO GIANT PLANET ORBITING VB10 5
very low-mass stars to search for planets. The results Seventh Framework Program as an International Fellow
from this work will help constrain the nature of planets (grantno.PIFF-GA-2009-234866). A.S.acknowledgesfi-
around the most numerous stars in the Galaxy. nancialsupportfromtheNSFundergrantAST-0708074.
H.H. acknowledges funding from the Swedish Research
Council (VR). H.N. acknowledges the financial support
We thank the ESO staff for their assistance with our
from the Lund Laser Center through a Linnaeus grant
program. This work was partly supported by the DFG
from the Swedish Research Council (VR). A.R. received
through grants GRK 1351 and RE 1664/4-1, and the
support from the DFG as an Emmy Noether Fellow.
BMBF through program 05A0GU2. J.L.B. acknowl-
Facilities: VLT:Antu (CRIRES)
edges research funding from the European Commissions
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