Free-floating Planetary-mass Objects Research Articles

Microlensing due to free-floating moon-planet systems

ABSTRACT Gravitational microlensing is a powerful method for detecting and characterizing free-floating planetary-mass objects (FFPs). FFPs could have exomoons rotating them. In this work, we study the probability of realizing these systems (i.e. free-floating moon-planet ones) through microlensing observations. These systems make mostly close caustic configurations with a considerable finite-source effect. We investigate finite-source microlensing light curves owing to free-floating moon-planet systems. We conclude that crossing planetary caustics causes an extensive extra peak at light curves’ wing that only changes its width if the source star does not cross the central caustic. If the source trajectory is normal to the moon-planet axis, the moon-induced perturbation has a symmetric shape with respect to the magnification peak, and its light curve is similar to a single-lens one with a higher finite-source effect. We evaluate the Roman efficiency for realizing moon-induced perturbations, which is $\left[0.002-0.094\right]\ \mathrm{ per\, cent}$ by assuming a log-uniform distribution for moon-planet mass ratio in the range ∈ [ −9, −2]. The highest detection efficiency (i.e. $\simeq 0.094~{{\ \rm per\ cent}}$) happens for Saturn-mass planets when moon-planet distance is ∼43Rp, where Rp is the Saturn radius. Enhancing planetary mass extends the event’s time-scale and decreases the finite-source effect, but it reduces the projected moon-planet distance normalized to the Einstein radius s(RE) which in turn decreases the size of planetary caustics and takes them away from the host planet’s position in close caustic configurations.

The TEMPO Survey. I. Predicting Yields of Transiting Exosatellites, Moons, and Planets from a 30 days Survey of Orion with the Roman Space Telescope

We present design considerations for the Transiting Exosatellites, Moons, and Planets in Orion (TEMPO) Survey with the Nancy Grace Roman Space Telescope. This proposed 30 days survey is designed to detect a population of transiting extrasolar satellites, moons, and planets in the Orion Nebula Cluster (ONC). The young (1–3 Myr), densely populated ONC harbors about a thousand bright brown dwarfs (BDs) and free-floating planetary-mass objects (FFPs). TEMPO offers sufficient photometric precision to monitor FFPs with M >1 M J for transiting satellites. The survey is also capable of detecting FFPs down to sub-Saturn masses via direct imaging, although follow-up confirmation will be challenging. TEMPO yield estimates include 14 (3–22) exomoons/satellites transiting FFPs and 54 (8–100) satellites transiting BDs. Of this population, approximately 50% of companions would be “super-Titans” (Titan to Earth mass). Yield estimates also include approximately 150 exoplanets transiting young Orion stars, of which >50% will orbit mid-to-late M dwarfs. TEMPO would provide the first census demographics of small exosatellites orbiting FFPs and BDs, while simultaneously offering insights into exoplanet evolution at the earliest stages. This detected exosatellite population is likely to be markedly different from the current census of exoplanets with similar masses (e.g., Earth-mass exosatellites that still possess H/He envelopes). Although our yield estimates are highly uncertain, as there are no known exoplanets or exomoons analogous to these satellites, the TEMPO survey would test the prevailing theories of exosatellite formation and evolution, which limit the certainty surrounding detection yields.

Rogue Planets and Brown Dwarfs: Predicting the Populations Free-floating Planetary Mass Objects Observable with JWST

Free-floating (or rogue) planets are planets that are liberated (or ejected) from their host systems. Although simulations predict their existence in substantial numbers, direct observational evidence for free-floating planets with masses below ∼5 M Jup is still lacking. Several cycle-1 observing programs with JWST aim to hunt for them in four different star-forming clusters. These surveys are designed to be sensitive to masses of 1–15 M Jup (assuming a hot-start formation), which corresponds to spectral types of early L to late T for the ages of these clusters. If the existing simulations are not wide off the mark, we show here that the planned programs are likely to find up to 10–20 giant rogue planets in moderate density clusters like NGC1333 or IC348, and several dozen to ∼100 in high-density regions like NGC2024 and the Orion Nebula Cluster. These numbers correspond to 1%–5% of the total cluster population; they could be substantially higher if stars form multiple giant planets at birth. In contrast, the number of free-floating brown dwarfs, formed from core collapse (like stars) is expected to be significantly lower, only about 0.25% of the number of stars, or 1–7 for the clusters considered here. Below 10 M Jup that number drops further by an order of magnitude. We also show that the planned surveys are not at risk of being significantly contaminated by field brown dwarfs in the foreground or background, after spectroscopic confirmation. Taken together, our results imply that if a population of L and T dwarfs were to be found in these JWST surveys, it is expected to be predominantly made up of rogue planets.

Atmospheric circulation of brown dwarfs and directly imaged exoplanets driven by cloud radiative feedback: effects of rotation

ABSTRACT Observations of brown dwarfs (BDs), free-floating planetary-mass objects, and directly imaged extrasolar giant planets (EGPs) exhibit rich evidence of large-scale weather. Cloud radiative feedback has been proposed as a potential mechanism driving the vigorous atmospheric circulation on BDs and directly imaged EGPs, and yet it has not been demonstrated in three-dimensional dynamical models at relevant conditions. Here, we present a series of atmospheric circulation models that self-consistently couple dynamics with idealized cloud formation and its radiative effects. We demonstrate that vigorous atmospheric circulation can be triggered and self-maintained by cloud radiative feedback. Typical isobaric temperature variation could reach over 100 K and horizontally averaged wind speed could be several hundreds of $\, {\rm m\, s^{-1}}$. The circulation is dominated by cloud-forming and clear-sky vortices that evolve over time-scales from several to tens of hours. The typical horizontal length-scale of dominant vortices is closed to the Rossby deformation radius, showing a linear dependence on the inverse of rotation rate. Stronger rotation tends to weaken vertical transport of vapour and clouds, leading to overall thinner clouds. Domain-mean outgoing radiative flux exhibits variability over time-scales of tens of hours due to the statistical evolution of storms. Different bottom boundary conditions in the models could lead to qualitatively different circulation near the observable layer. The circulation driven by cloud radiative feedback represents a robust mechanism generating significant surface inhomogeneity as well as irregular flux time variability. Our results have important implications for near-infrared (IR) colours of dusty BDs and EGPs, including the scatter in the near-IR colour–magnitude diagram and the viewing-geometry-dependent near-IR colours.

A Wide Planetary-mass Companion to a Young Low-mass Brown Dwarf in Ophiuchus

Abstract We present the discovery of a planetary-mass companion to CFHTWIR-Oph 98, a low-mass brown dwarf member of the young Ophiuchus star-forming region, with a wide 200 au separation (1.″46). The companion was identified using Hubble Space Telescope images, and confirmed to share common proper motion with the primary using archival and new ground-based observations. Based on the very low probability of the components being unrelated Ophiuchus members, we conclude that Oph 98 AB forms a binary system. From our multiband photometry, we constrain the primary to be an M9–L1 dwarf, and the faint companion to have an L2–L6 spectral type. For a median age of 3 Myr for Ophiuchus, fits of evolutionary models to measured luminosities yield masses of 15.4 ± 0.8 M Jup for Oph 98 A and 7.8 ± 0.8 M Jup for Oph 98 B, with respective effective temperatures of 2320 ± 40 K and 1800 ± 40 K. For possible system ages of 1–7 Myr, masses could range from 9.6–18.4 M Jup for the primary, and from 4.1–11.6 M Jup for the secondary. The low component masses and very large separation make this binary the lowest binding energy system imaged to date, indicating that the outcome of low-mass star formation can result in such extreme, weakly bound systems. With such a young age, Oph 98 AB extends the growing population of young free-floating planetary-mass objects, offering a new benchmark to refine formation theories at the lowest masses.

Predictions of the Nancy Grace Roman Space Telescope Galactic Exoplanet Survey. II. Free-floating Planet Detection Rates*

Abstract The Nancy Grace Roman Space Telescope (Roman) will perform a Galactic Exoplanet Survey (RGES) to discover bound exoplanets with semimajor axes greater than 1 au using gravitational microlensing. Roman will even be sensitive to planetary-mass objects that are not gravitationally bound to any host star. Such free-floating planetary-mass objects (FFPs) will be detected as isolated microlensing events with timescales shorter than a few days. A measurement of the abundance and mass function of FFPs is a powerful diagnostic of the formation and evolution of planetary systems, as well as the physics of the formation of isolated objects via direct collapse. We show that Roman will be sensitive to FFP lenses that have masses from that of Mars (0.1 M ⊕) to gas giants (M ≳ 100 M ⊕) as isolated lensing events with timescales from a few hours to several tens of days, respectively. We investigate the impact of the detection criteria on the survey, especially in the presence of finite-source effects for low-mass lenses. The number of detections will depend on the abundance of such FFPs as a function of mass, which is at present poorly constrained. Assuming that FFPs follow the fiducial mass function of cold, bound planets adapted from Cassan et al., we estimate that Roman will detect ∼250 FFPs with masses down to that of Mars (including ∼60 with masses ≤ M ⊕). We also predict that Roman will improve the upper limits on FFP populations by at least an order of magnitude compared to currently existing constraints.

No large population of unbound or wide-orbit Jupiter-mass planets.

Planet formation theories predict that some planets may be ejected from their parent systems as result of dynamical interactions and other processes. Unbound planets can also be formed through gravitational collapse, in a way similar to that in which stars form. A handful of free-floating planetary-mass objects have been discovered by infrared surveys of young stellar clusters and star-forming regions as well as wide-field surveys, but these studies are incomplete for objects below five Jupiter masses. Gravitational microlensing is the only method capable of exploring the entire population of free-floating planets down to Mars-mass objects, because the microlensing signal does not depend on the brightness of the lensing object. A characteristic timescale of microlensing events depends on the mass of the lens: the less massive the lens, the shorter the microlensing event. A previous analysis of 474 microlensing events found an excess of ten very short events (1-2 days)-more than known stellar populations would suggest-indicating the existence of a large population of unbound or wide-orbit Jupiter-mass planets (reported to be almost twice as common as main-sequence stars). These results, however, do not match predictions of planet-formation theories and surveys of young clusters. Here we analyse a sample of microlensing events six times larger than that of ref. 11 discovered during the years 2010-15. Although our survey has very high sensitivity (detection efficiency) to short-timescale (1-2 days) microlensing events, we found no excess of events with timescales in this range, with a 95 per cent upper limit on the frequency of Jupiter-mass free-floating or wide-orbit planets of 0.25 planets per main-sequence star. We detected a few possible ultrashort-timescale events (with timescales of less than half a day), which may indicate the existence of Earth-mass and super-Earth-mass free-floating planets, as predicted by planet-formation theories.

Near-infrared photometry of WISE J085510.74–071442.5

(Abridged) We aim at measuring the near-infrared photometry, and deriving the mass, age, temperature, and surface gravity of WISE J085510.74-071442.5 (J0855-0714), which is the coolest known object beyond the Solar System as of today. We use publicly available data from the archives of the HST and the VLT to determine the emission of this source at 1.153 micron (F110W) and 1.575 micron (CH_4). J0855-0714 is detected at both wavelengths with signal-to-noise ratio of ~10 (F110W) and ~4 (CH_4-off) at the peak of the corresponding PSFs. This is the first detection of J0855-0714 in the H-band. We measure 26.31 +/- 0.10 and 23.22 +/- 0.35 mag in F110W and CH_4 (Vega system). J0855-0714 remains unresolved in the HST images that have a spatial resolution of 0.22". Companions at separations of 0.5 AU (similar brightness) and at ~1 AU (~1 mag fainter in the F110W filter) are discarded. By combining the new data with published photometry, we build the spectral energy distribution of J0855-0714 from 0.89 to 22.09 micron, and contrast it against state-of-the-art solar-metallicity models of planetary atmospheres. We determine a temperature of 225-250 K, a bolometric luminosity of log L/Lsol = -8.57, and a high surface gravity of log g = 5.0 (cm/s2), which suggests an old age although such a high gravity is not fully compatible with evolutionary models. After comparison with the cooling theory for brown dwarfs and planets, we infer a mass in the interval 2-10 Mjup for ages of 1-12 Gyr and log g > 3.5 (cm/s2). At the age of the Sun, J0855-0714 would be a ~5-Mjup free-floating planetary-mass object. J0855-0714 may represent the old image of the free-floating planetary-mass objects of similar mass discovered in star-forming regions and young stellar clusters. As many J0855-0714-like objects as M5-L2 stars may be expected to populate the solar neighborhood.

VARIABILITY IN A YOUNG, L/T TRANSITION PLANETARY-MASS OBJECT

As part of our ongoing NTT SoFI survey for variability in young free-floating planets and low mass brown dwarfs, we detect significant variability in the young, free-floating planetary mass object PSO J318.5-22, likely due to rotational modulation of inhomogeneous cloud cover. A member of the 23$\pm$3 Myr $\beta$ Pic moving group, PSO J318.5-22 has T$_\mathrm{eff}$ = 1160$^{+30}_{-40}$ K and a mass estimate of 8.3$\pm$0.5 M$_{Jup}$ for a 23$\pm$3 Myr age. PSO J318.5-22 is intermediate in mass between 51 Eri b and $\beta$ Pic b, the two known exoplanet companions in the $\beta$ Pic moving group. With variability amplitudes from 7-10$\%$ in J$_{S}$ at two separate epochs over 3-5 hour observations, we constrain the rotational period of this object to $>$5 hours. In K$_{S}$, we marginally detect a variability trend of up to 3$\%$ over a 3 hour observation. This is the first detection of weather on an extrasolar planetary mass object. Among L dwarfs surveyed at high-photometric precision ($<$3$\%$) this is the highest amplitude variability detection. Given the low surface gravity of this object, the high amplitude preliminarily suggests that such objects may be more variable than their high mass counterparts, although observations of a larger sample is necessary to confirm this. Measuring similar variability for directly imaged planetary companions is possible with instruments such as SPHERE and GPI and will provide important constraints on formation. Measuring variability at multiple wavelengths can help constrain cloud structure.

Search for free-floating planetary-mass objects in the Pleiades

(Abridged) We aim at identifying the least massive population of the solar metallicity, young (120 Myr), nearby (133.5 pc) Pleiades star cluster with the ultimate goal of understanding the physical properties of intermediate-age, free-floating, low-mass brown dwarfs and giant planetary-mass objects, and deriving the cluster substellar mass function across the deuterium-burning mass limit at ~0.012 Msol. We performed a deep photometric and astrometric J- and H-band survey covering an area of ~0.8 deg^2. The images with completeness and limiting magnitudes of J,H ~ 20.2 and ~ 21.5 mag were acquired ~9 yr apart (proper motion precision of +/-6 mas/yr). J- and H-band data were complemented with Z, K, and mid-infrared magnitudes up to 4.6 micron coming from UKIDSS, WISE, and follow-up observations of our own. Pleiades member candidates were selected to have proper motions compatible with that of the cluster, and colors following the known Pleiades sequence in the interval J = 15.5-8.8 mag, and Z_UKIDSS - J > 2.3 mag or Z nondetections for J > 18.8 mag. We found a neat sequence of astrometric and photometric Pleiades substellar member candidates in the intervals J = 15.5-21.2 mag and ~0.072-0.008 Msol. The faintest objects show very red near- and mid-infrared colors exceeding those of field high-gravity dwarfs by >0.5 mag. The Pleiades photometric sequence does not show any color turn-over because of the presence of photospheric methane absorption down to J = 20.3 mag, which is about 1 mag fainter than predicted by the color-computed models. Pleiades brown dwarfs have a proper motion dispersion of 6.4-7.5 mas/yr and are dynamically relaxed at the age of the cluster. The Pleiades mass function extends down to the deuterium burning-mass threshold, with a slope fairly similar to that of other young star clusters and stellar associations.

The tiny globulettes in the Carina nebula

Small molecular cloudlets are abundant in many H II regions surrounding newborn stellar clusters. In optical images these so-called globulettes appear as dark silhouettes against the bright nebular background. We have located close to 300 globulettes in the Carina Nebula. The objects appear as well-confined dense clumps and, as a rule, lack thinner envelopes, bright rims, and tails. Some globulettes are slightly elongated with their major axes oriented in the direction of young clusters in the complex. Many objects are quite isolated and reside at projected distances >1.5 pc from other molecular structures in the region. No globulette coincides in position with recognized pre-main-sequence objects in the area. The objects are systematically much smaller, less massive, and much denser than those surveyed in other H II regions. Practically all globulettes are of planetary mass, and most have masses less than one Jupiter mass. The average number densities exceed 10^5 cm^-3 in several objects. We have found a statistical relation between density and radius (mass) in the sense that the smallest objects are also the densest. The population of small globulettes in Carina appears to represent a more advanced evolutionary state than those investigated in other H II regions. The objects are subject to erosion in the intense radiation field, which would lead to a removal of any thinner envelope and an unveiling of the core, which becomes more compact with time. We discuss the possibility that the core may become gravitationally unstable, in which case free-floating planetary mass objects can form.

BANYAN. II. VERY LOW MASS AND SUBSTELLAR CANDIDATE MEMBERS TO NEARBY, YOUNG KINEMATIC GROUPS WITH PREVIOUSLY KNOWN SIGNS OF YOUTH

We present Bayesian Analysis for Nearby Young AssociatioNs II (BANYAN II), a modified Bayesian analysis for assessing the membership of later-than-M5 objects to any of several Nearby Young Associations (NYAs). In addition to using kinematic information (from sky position and proper motion), this analysis exploits 2MASS-WISE color-magnitude diagrams in which old and young objects follow distinct sequences. As an improvement over our earlier work, the spatial and kinematic distributions for each association are now modelled as ellipsoids whose axes need not be aligned with the Galactic coordinate axes, and we use prior probabilities matching the expected populations of the NYAs considered versus field stars. We present an extensive contamination analysis to characterize the performance of our new method. We find that Bayesian probabilities are generally representative of contamination rates, except when a parallax measurement is considered. In this case contamination rates become significantly smaller and hence Bayesian probabilities for NYA memberships are pessimistic. We apply this new algorithm to a sample of 158 objects from the literature that are either known to display spectroscopic signs of youth or have unusually red near-infrared colors for their spectral type. Based on our analysis, we identify 25 objects as new highly probable candidates to NYAs, including a new M7.5 bona fide member to Tucana-Horologium, making it the latest-type member. In addition, we reveal that a known L2{\gamma} dwarf is co-moving with a bright M5 dwarf, and we show for the first time that two of the currently known ultra red L dwarfs are strong candidates to the AB Doradus moving group. Several objects identified here as highly probable members to NYAs could be free-floating planetary-mass objects if their membership is confirmed.

SUBSTELLAR OBJECTS IN NEARBY YOUNG CLUSTERS (SONYC). VI. THE PLANETARY-MASS DOMAIN OF NGC 1333

Within the SONYC - Substellar Objects in Nearby Young Clusters - survey, we investigate the frequency of free-floating planetary-mass objects (planemos) in the young cluster NGC1333. Building upon our extensive previous work, we present spectra for 12 of the faintest candidates from our deep multi-band imaging, plus seven random objects in the same fields, using MOIRCS on Subaru. We confirm seven new sources as young very low mass objects (VLMOs), with Teff of 2400-3100K and mid-M to early-L spectral types. These objects add to the growing census of VLMOs in NGC1333, now totaling 58. Three confirmed objects (one found in this study) have masses below 15 MJup, according to evolutionary models, thus are likely planemos. We estimate the total planemo population with 5-15 MJup in NGC1333 is <~8. The mass spectrum in this cluster is well approximated by dN/dM ~ M^-alpha, with a single value of alpha = 0.6+/-0.1 for M<0.6Msol, consistent with other nearby star forming regions, and requires alpha <~ 0.6 in the planemo domain. Our results in NGC1333, as well as findings in several other clusters by ourselves and others, confirm that the star formation process extends into the planetary-mass domain, at least down to 6 MJup. However, given that planemos are 20-50 times less numerous than stars, their contribution to the object number and mass budget in young clusters is negligible. Our findings disagree strongly with the recent claim from a microlensing study that free-floating planetary-mass objects are twice as common as stars - if the microlensing result is confirmed, those isolated Jupiter-mass objects must have a different origin from brown dwarfs and planemos observed in young clusters.

Direct imaging constraints on planet populations detected by microlensing

Results from gravitational microlensing suggested the existence of a large population of free-floating planetary mass objects. The main conclusion from this work was partly based on constraints from a direct imaging survey. This survey determined upper limits for the frequency of stars that harbor giant exoplanets at large orbital separations. Aims. We want to verify to what extent upper limits from direct imaging do indeed constrain the microlensing results. We examine the current derivation of the upper limits used in the microlensing study and re-analyze the data from the corresponding imaging survey. We focus on the mass and semi-major axis ranges that are most relevant in context of the microlensing results. We also consider new results from a recent M-dwarf imaging survey as these objects are typically the host stars for planets detected by microlensing. We find that the upper limits currently applied in context of the microlensing results are probably underestimated. This means that a larger fraction of stars than assumed may harbor gas giant planets at larger orbital separations. Also, the way the upper limit is currently used to estimate the fraction of free-floating objects is not strictly correct. If the planetary surface density of giant planets around M-dwarfs is described as df_Planet ~ a^beta da, we find that beta ~ 0.5 - 0.6 is consistent with results from different observational studies probing semi-major axes between ~0.03 - 30 AU. Having a higher upper limit on the fraction of stars that may have gas giant planets at orbital separations probed by the microlensing data implies that more of the planets detected in the microlensing study are potentially bound to stars rather than free-floating. The current observational data are consistent with a rising planetary surface density for giant exoplanets around M-dwarfs out to ~30 AU.

Unbound or distant planetary mass population detected by gravitational microlensing

Since 1995, more than 500 exoplanets have been detected using different techniques, of which 12 were detected with gravitational microlensing. Most of these are gravitationally bound to their host stars. There is some evidence of free-floating planetary-mass objects in young star-forming regions, but these objects are limited to massive objects of 3 to 15 Jupiter masses with large uncertainties in photometric mass estimates and their abundance. Here, we report the discovery of a population of unbound or distant Jupiter-mass objects, which are almost twice (1.8(+1.7)(-0.8)) as common as main-sequence stars, based on two years of gravitational microlensing survey observations towards the Galactic Bulge. These planetary-mass objects have no host stars that can be detected within about ten astronomical units by gravitational microlensing. However, a comparison with constraints from direct imaging suggests that most of these planetary-mass objects are not bound to any host star. An abrupt change in the mass function at about one Jupiter mass favours the idea that their formation process is different from that of stars and brown dwarfs. They may have formed in proto-planetary disks and subsequently scattered into unbound or very distant orbits.

SEARCH FOR VERY LOW-MASS BROWN DWARFS AND FREE-FLOATING PLANETARY-MASS OBJECTS IN TAURUS

The number of low-mass brown dwarfs and even free floating planetary mass objects in young nearby star-forming regions and associations is continuously increasing, offering the possibility to study the low-mass end of the IMF in greater detail. In this paper, we present six new candidates for (very) low-mass objects in the Taurus star-forming region one of which was recently discovered in parallel by Luhman et al. (2009). The underlying data we use is part of a new database from a deep near-infrared survey at the Calar Alto observatory. The survey is more than four magnitudes deeper than the 2MASS survey and covers currently ~1.5 square degree. Complementary optical photometry from SDSS were available for roughly 1.0 square degree. After selection of the candidates using different color indices, additional photometry from Spitzer/IRAC was included in the analysis. In greater detail we focus on two very faint objects for which we obtained J-band spectra. Based on comparison with reference spectra we derive a spectral type of L2+/-0.5 for one object, making it the object with the latest spectral type in Taurus known today. From models we find the effective temperature to be 2080+/-140 K and the mass 5-15 Jupiter masses. For the second source the J-band spectrum does not provide a definite proof of the young, low-mass nature of the object as the expected steep water vapor absorption at 1.33 micron is not present in the data. We discuss the probability that this object might be a background giant or carbon star. If it were a young Taurus member, however, a comparison to theoretical models suggests that it lies close to or even below the deuterium burning limit (<13 Jupiter masses) as well. A first proper motion analysis for both objects shows that they are good candidates for being Taurus members.

Globulettes as Seeds of Brown Dwarfs and Free-Floating Planetary-Mass Objects

Some H II regions surrounding young stellar clusters contain tiny dusty clouds, which on photos look like dark spots or teardrops against a background of nebular emission. From our collection of H? images of 10 H II regions gathered at the Nordic Optical Telescope, we found 173 such clouds, which we call globulettes, since they are much smaller than normal globules and form a distinct class of objects. Many globulettes are quite isolated and located far from the molecular shells and elephant trunks associated with the regions. Others are attached to the trunks (or shells), suggesting that globulettes may form as a consequence of erosion of these larger structures. None of our objects appear to contain stellar objects. The globulettes were measured for position, dimension, and orientation, and we find that most objects are smaller than 10 kAU. The Rosette Nebula and IC 1805 are particularly rich in globulettes, for which the size distributions peak at mean radii of ~2.5 kAU, similar to what was found by Reipurth and coworkers and De Marco and coworkers for similar objects in other regions. We estimate total mass and density distributions for each object from extinction measures and conclude that a majority contain <13 MJ, corresponding to planetary-mass objects. We then estimate the internal thermal and potential energies and find, when also including the effects from the outer pressure, that a large fraction of the globulettes could be unstable and would contract on short timescales, <10 6 yr. In addition, the radiation pressure and ram pressure exerted on the side facing the clusters would stimulate contraction. Since the globulettes are not screened from stellar light by dust clouds farther in, one would expect photoevaporation to dissolve the objects. However, surprisingly few objects show bright rims or teardrop forms. We calculate the expected lifetimes against photoevaporation. These lifetimes scatter around 4 ? 106 yr, much longer than estimated in previous studies and also much longer than the free-fall time. We conclude that a large number of our globulettes have time to form central low-mass objects long before the ionization front, driven by the impinging Lyman photons, has penetrated far into the globulette. Hence, the globulettes may be one source in the formation of brown dwarfs and free-floating planetary-mass objects in the galaxy.

Spectroscopy of Young Planetary Mass Candidates with Disks

It is now well established that many young brown dwarfs exhibit characteristics similar to classical T Tauri stars, including infrared excess from disks and emission lines related to accretion. Whether the same holds true for even lower mass objects, namely, those near and below the Deuterium-burning limit, is an important question. Here we present optical spectra of six isolated planetary mass candidates in Chamaeleon II, Lupus I, and Ophiuchus star-forming regions, recently identified by Allers and collaborators to harbor substantial mid-infrared excesses. Our spectra, from ESO's Very Large Telescope and New Technology Telescope, show that four of the targets have spectral types in the ~M9-L1 range, and three of those also exhibit Hα. Their luminosities are consistent with masses of ~5MJ-15MJ according to models of Baraffe and coworkers, thus placing these four objects among the lowest mass brown dwarfs known to be surrounded by circum-substellar disks. Our findings bolster the idea that free-floating planetary mass objects could have infancies remarkably similar to those of Sun-like stars and suggest the intriguing possibility of planet formation around primaries whose masses are comparable to those of extrasolar giant planets. Another target appears to be a brown dwarf (~M8) with prominent Hα emission, possibly arising from accretion. The sixth candidate is likely a background source, underlining the need for spectroscopic confirmation.

On the origin of brown dwarfs and free-floating planetary-mass objects

Briceno et al. report a significantly smaller number of brown dwarfs (BDs) per star in the Taurus-Auriga (TA) pre-main sequence stellar groups than in the central region of the Orion Nebula cluster (ONC). Also, BDs have binary properties that are not compatible with a star-like formation history. It is shown here that these results can be understood if BDs are produced as ejected embryos with a dispersion of ejection velocities of about 2 km/s and if the number of ejected embryos is about one per four stars born in TA and ONC. The Briceno et al. observation is thus compatible with a universal BD production mechanism and a universal IMF, but the required number of BDs per star is much too small to account for the one BD per star deduced to be present in the Galactic field. There are two other mechanisms for producing BDs and free-floating planetary-mass objects (FFLOPs), namely the removal of accretion envelopes from low-mass proto-stars through photo-evaporation through nearby massive stars, and hyperbolic collisions between proto-stars in dense clusters. The third BD flavour, the collisional BDs, can be neglected in the ONC. It is shown that the observed IMF with a flattening near 0.5 M⊙ can be re-produced via photo-evaporation of proto-stars if they are distributed according to a featureless Salpeter MF above the sub-stellar mass limit, and that the photo-evaporated BDs should have a smaller velocity dispersion than the stars. The number of photo-evaporated BDs per star should increase with cluster mass, peaking in globular clusters that would have contained many stars as massive as 150 M⊙. The required number of embryo-ejected BDs in TA and the ONC can be as low as 6 ejected BDs per 100 stars if the central ONC contains 0.23 photo-evaporated BDs per star. Alternatively, if the assumption is discarded that embryo ejection must operate equally in all environments, then it can be argued that TA produced about one ejected BD per star leading to consistency with the Galacticfield observations. The dispersion of ejection velocities would be about 3 km/s. In the central ONC the number of ejected BDs per star would then be at most 0.37, or less if photo-evaporated BDs contribute. This non-universal scenario would thus imply that the Galactic-field BD population may mostly stem from TA-like star formation or modest clusters, the ONC not being able to contribute more than about 0.25± 0.04 BDs per star.

The Substellar Mass Function in σ Orionis

We combine results from imaging searches for substellar objects in the sigma Orionis cluster and follow-up photometric and spectroscopic observations to derive a census of the brown dwarf population in a region of 847 arcmin^2. We identify 64 very low-mass cluster member candidates in this region. We have available three color (IZJ) photometry for all of them, spectra for 9 objects, and K photometry for 27% of our sample. These data provide a well defined sequence in the I vs I-J, I-K color magnitude diagrams, and indicate that the cluster is affected by little reddening despite its young age (~5 Myr). Using state-of-the-art evolutionary models, we derive a mass function from the low-mass stars (0.2 Msol) across the complete brown dwarf domain (0.075 Msol to 0.013 Msol), and into the realm of free-floating planetary-mass objects (<0.013 Msol). We find that the mass spectrum (dN/dm ~ m^{-alpha}) increases toward lower masses with an exponent alpha = 0.8+/-0.4. Our results suggest that planetary-mass isolated objects could be as common as brown dwarfs; both kinds of objects together would be as numerous as stars in the cluster. If the distribution of stellar and substellar masses in sigma Orionis is representative of the Galactic disk, older and much lower luminosity free-floating planetary-mass objects with masses down to about 0.005 Msol should be abundant in the solar vicinity, with a density similar to M-type stars.