Mon. Not. R. Astron. Soc. (2012) doi:10.1111/j.1365-2966.2012.21009.x
Nomads of the Galaxy
Louis E. Strigari,1 Matteo Barnabe`,1 Philip J. Marshall2 and Roger D. Blandford11Kavli Institute for Particle Astrophysics and Cosmology, Stanford University, Stanford, CA 94305, USA2Department of Physics, University of Oxford, Keble Road, Oxford OX1 3RH
Accepted 2012 March 28. Received 2012 March 26; in original form 2012 January 25
ABSTRACTWe estimate that there may be up to 105 compact objects in the mass range 108102 Mper-main-sequence star that are unbound to a host star in the Galaxy. We refer to these objectsas nomads; in the literature a subset of these are sometimes called free-floating or rogue planets.Our estimate for the number of Galactic nomads is consistent with a smooth extrapolation ofthe mass function of unbound objects above the Jupiter-mass scale, the stellar mass densitylimit and the metallicity of the interstellar medium. We analyse the prospects for detectingnomads via Galactic microlensing. The Wide-Field Infrared Survey Telescope will measurethe number of nomads per-main-sequence star greater than the mass of Jupiter to 13 per cent,and the corresponding number greater than the mass of Mars to 25 per cent. All-sky surveyssuch as Gaia and Large Synoptic Survey Telescope can identify nomads greater than aboutthe mass of Jupiter. We suggest a dedicated drift scanning telescope that covers approximately100 deg2 in the Southern hemisphere could identify nomads via microlensing of bright starswith characteristic time-scales of tens to hundreds of seconds.Key words: gravitational lensing: micro planets and satellites: detection planets andsatellites: general.
1 IN T RO D U C T I O N
The recent years have witnessed a rapid rise in the number of knownplanetary mass objects,
Nomads of the Galaxy 3
Figure 1. Number of nomads greater than a given mass scale, Nnm(> M),relative to the number of main-sequence stars, NMS. Three different slopesfor the nomad mass function are labelled, nm = 2, 1.3, 0.5. The upper blackcurve is determined assuming that objects at that mass scale have a densityof 0 = 0.1 M pc3.
mmin = 105 M implies 50. Extrapolation down to mmin =106 M implies an order of magnitude increase in 700, whileextrapolation down to mmin = 108 M implies 105.
The above estimates indicate that, when fixing to the measuredabundance of nomads greater than 103 M and smoothly extrap-olating to lower masses, there is several orders of magnitude uncer-tainty on the nomad abundance. For an appropriately large ratio ofthe number of nomads to main-sequence stars, the nomad mass func-tion is constrained by the limits on the mass density of the Galacticdisc, which is 0 = 0.1 M pc3 (Holmberg & Flynn 2000). As anexample, if we assume that the entire local mass distribution of thedisc is composed of objects at the mass scale 108 M, the bound0 = 0.1 M pc3 corresponds to an upper limit of 106 compactobjects per-main-sequence star. Masses of compact objects at thesescales and below may be probed by future short cadence microlens-ing observations with the Kepler satellite (see Griest et al. 2011,and the discussion below).
Predictions for the number of nomads, in comparison to con-straints on the local mass density, are summarized in Fig. 1. Thisshows the number of nomads greater than a given mass scale,N( >M), relative to the number of main-sequence stars, NMS. Threedifferent slopes for the nomad mass function are labelled, nm =2, 1.3, 0.5, which have corresponding values for the slope of thebrown dwarf mass function of bd = 0, 0.5, 1. The upper limit,indicated as the diagonal line, is determined assuming that objectsat that mass scale have a density of 0 = 0.1 M pc3. For nm lessthan(greater than) 1, most of the number is in the high(low)-massend of the distribution, while for nm less than(greater than) 2 mostof the mass is in the high(low)-mass end of the distribution.
3 EV E N T R AT E S
In this section we calculate the microlensing event rates from thenomad population. We begin by establishing the definitions andthe Galactic model parameters, and then use this model to predictthe time-scale distribution of events and the integrated number ofevents detectable.
We employ standard microlensing formalism. The distance to thesource star is DS, the distance to the lens is DL and the mass of thelens is M. The Einstein radius is R2E = (4G/c2)MDL(DSDL)/DS,and the Einstein crossing time-scale is tE = RE/v. The amplificationof a source star is A(u) = (u2 + 2)/(uu2 + 4), where u is theprojected separation of the lens and source in units of the Einsteinradius.
To calculate microlensing event rates we use Galactic model 1of Rahal et al. (2009). This is characterized by an exponential thindisc with a scale length of Rd = 3.5 kpc and scaleheight of zh =0.325 kpc,
d(R) = 0 exp [(R R0)/Rd |z|/zh] , (1)where R0 = 8.5 kpc, zh = 0.35 kpc. We use the bulge density distri-bution from Dwek et al. (1995). The lenssource transverse velocitydistribution, f (vl, vb), is modelled as in Han & Gould (1996), withv =
v2l + v2b , where we indicate as vl and vb, respectively, the
velocity along the galactic longitude and latitude coordinates.We determine the total event rate distribution by breaking the
lensessources into the discbulge, bulgebulge and discdisc com-ponents. We consider two different targets of source stars. First,bulge stars in the direction of Baades window, (b, l) = (3.9,1), and secondly, sources distributed over all sky. For the bulgeobservations we can compare to observational determinations ofthe optical depth (Sumi et al. 2003, 2006; Popowski et al. 2005;Hamadache et al. 2006) and to the theoretical optical depth cal-culations (Han & Gould 2003; Wood & Mao 2005) by smoothlyextrapolating the rates from the nomad mass regime to the massregime of main-sequence stars and remnants.
3.2 Finite source effects
Since we extrapolate the nomad mass function down to low massscale, it is important to properly account for finite source effects inthe microlensing events. More specifically, we need to estimate byhow much the peak amplification of an event is reduced when REis of order the projected radius of the source star. To estimate finitesource effects we take the source stars to have uniform surfacebrightness, and for a given projected lenssource separation weestimate the amplification as
Afs(u) = 12
u2 + 2 2u cos ), (2)
where = R/RE. For typical lens and source distances, DL 6.5 kpc and DS 8 kpc (Zhao, Spergel & Rich 1995), the ampli-fication of a main-sequence source star for several different massnomads is plotted in Fig. 2. This figure indicates that, for the as-sumed Galactic model, lenses down to the mass of a few times107 M can be probed at the most probable distance. For thelensing of a giant star at these same distances, the limiting massis more near the Jupiter mass. However, since in our analysis weintegrate over the entire possible range of lens and source distances,it is possible to quantify the probability of detecting a lens that ismore nearby and less massive than the lunar mass. We address thispossibility in more detail in Section 7.
3.3 Bulge event rate
The microlensing event rate per source star in a direction (b, l)is given by an integral over the lenssource transverse velocity
C 2012 The AuthorsMonthly Notices of the Royal Astronomical Society C 2012 RAS
4 L. E. Strigari et al.
Figure 2. Amplification light curves of main-sequence source stars at 8 kpcfor nomads of different masses at all a distance of 6.5 kpc. A uniform surfacebrightness for the star has been assumed.
distribution, the lens density distributions L and the mass function(Griest 1991; Kiraga & Paczynski 1994):d(b, l)
dvl dvbvf (vb, vl)(tE RE/v)
dM (M)REL(l, b,DL). (3)
The mass function (M) is normalized to the mean mass of thelens population. The optical depth is tE/2, where isthe integral of the event rate distribution over all tE. In equation (3),uT = (A2/
A2 11)1/2, with A = 1.34 corresponding to uT = 1.
This corresponds to the event rate for source stars within a circulararea of one Einstein radius of the lens star. This is a conservativecriteria that is appropriate for our analysis; event rates are increasedfor uT > 1 when allowing for A < 1.34.
Fig. 3 shows the event rate distributions in the direction of thebulge, with each panel corresponding to a different assumption
for the slope of the nomad mass function. In all panels we take1 = 2.0 and 2 = 1.3 for the main-sequence stellar mass function.Within each of the panels there are three different assumptionsfor mmin. The three curves in all of the panels represent the sumof the event rate distribution from bulge and disc lenses. For allcurves in the middle and right-hand panels, the mean time-scaleof a microlensing event is tE 50 d. However, for the curvesin the left-hand panel, tE depends strongly on mmin because ofthe steep power law to low masses. Specifically for mmin = (102,105, 108) M, the respective mean time-scales are tE (50, 35,3) d. For all curves, the optical depths are 1.5 106, consistentwith the theoretical calculations above and the observations. Theinclusion of the nomad population does not affect the total opticaldepth because this