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we have witnessed in the last decade the detection of free - floating substellar objects with increasingly lower masses and cooler temperatures that reach the canonical planetary regime of a few jupiter masses and a few hundred kelvin .
examples are free - floating planetary mass objects in star - forming regions ( e.g. , @xcite ) and ultracool y dwarfs in the field ( e.g. , @xcite ) .
one of the main open questions in the theory of star formation is : how do these free - floating planetary - like objects - and brown dwarfs in general - form ?
a high - density phase is necessary for the gravitational fragmentation to create very small jeans - unstable cores .
proposed scenarios to prevent a substellar core in a dense environment from accreting to stellar mass are ejection of the core through dynamical interactions ( e.g. , @xcite ) or photo - evaporation of the gas envelope through radiation of a nearby hot star ( e.g. , ) . also suggested was the formation of brown dwarfs by disk instabilities in the outer regions of massive circumstellar disks ( e.g. , @xcite ) .
alternatively , brown dwarfs could form in a more isolated mode by direct collapse .
for example , filament collapse ( e.g. , inutsuka & miyama 1992 ) might form low - mass cores that experience high self - erosion in outflows and become brown dwarfs ( machida et al .
a key to understanding star and brown dwarf formation is to observationally define the minimum mass that the canonical star - like mode of formation can produce by detecting and exploring the main features characteristic of this process , such as disks , accretion , outflows , and orbiting planets , for the lowest - mass objects .
the first indications for the existence of free - floating planetary mass objects came from observations in the very young @xmath2 orionis cluster @xcite . while it is difficult to establish the membership to @xmath2 ori and , therefore , the age and mass , some very promising candidates were detected .
an example is sori60 , for which mid - ir excess indicates a young age @xcite .
other candidates , however , turned out not to be members of the cluster , such as sori70 ( @xcite ; pea ramirez , this volume ) .
the coolest known free - floating objects are y dwarfs , of which 17 are spectroscopically confirmed to date @xcite . while we can expect many planetary mass objects among y dwarfs , the age and mass of these nearby field objects are in most cases unknown . the first confirmed y dwarf , wise1828 + 2650 @xcite , was suggested to have a temperature and mass of only about 300k and 3 - 6@xmath3 , respectively , although there are indications that the temperature might be significantly larger @xcite . very recently
a remarkably cold and low - mass object was detected in the very near vicinity of our sun : wise0855 - 0714 at a distance of 2.2pc has a proposed temperature and mass of 250k and 3 - 10@xmath3 , respectively @xcite .
nearby objects that do allow constraints on their age and , therefore , mass are members of young moving groups ( see also faherty , this volume ) .
one of them , psoj318.5 - 22 , was recently detected to be a planetary - mass member of @xmath1 pic .
depending on the assumed age for @xmath1 pic its mass is estimated to 6.5@xmath3 ( age : 12myr ) and 8.4@xmath3 ( 21myr ) , respectively ( @xcite ; allers , this volume ) .    to address the question how free - floating planets form , we observed ots44 , the lowest mass known member of the chamaeleon  i star - forming region ( chai , @xmath02myr , 160pc ) .
ots44 was first identified as a brown dwarf candidate in a deep near - ir imaging survey @xcite and later confirmed as very low - mass substellar object of spectral - type m9.5 based on low - resolution near - ir and optical spectra ( e.g. * ? ? ?
its mass was estimated using near - ir spectra to lie in or very close to the planetary regime ( @xmath06 - 17m@xmath4 , ) with an average rounded value of 12m@xmath4 .
mid - ir photometry of ots44 by spitzer / irac and mips @xcite indicated the presence of circumstellar material surrounding ots44 .
recently , the disk of ots44 was detected at far - ir wavelength by herschel / pacs @xcite .
we re - analyzed the herschel flux measurement @xcite and find a slightly smaller value for ots44 than @xcite .
we modeled the sed of ots44 based on flux measurements from the optical to the far - ir using the radiative transfer code ` mc3d ` @xcite .
we employ a passive - disk model consisting of a central substellar source surrounded by a parameterized flared disk in which dust and gas are well mixed and homogeneous throughout the system .
we assume dust grains of astronomical silicate ( 62.5% ) and graphite ( 37.5% ) with minimum and maximum grain sizes of 0.005@xmath5 and 0.25@xmath5 , respectively .
as there are model degeneracies between different disk parameters , we conducted a bayesian analysis to estimate the validity range for each parameter .
see for details .
fig.[fig : sed ] shows the sed and the best - fit disk model of ots44 based on the re - analysis of the herschel fluxes @xcite .
we find that ots44 has a highly flared disk ( @xmath61.3 ) with a disk mass of 3.25@xmath7 , i.e. about @xmath8 .
sed of ots44 . shown
are photometric measurements ( red diamonds ) an upper limit for the 160@xmath9 flux ( black triangle ) , the best - fit sed model ( blue thick line ) , and the input bt - settl photosphere model ( gray dotted line ) .
this is a slightly revised version of the sed model of ots44 in using a revised herschel flux measurement @xcite.,width=321 ]      we took near - ir j - band spectra ( 1.1 - 1.4 @xmath10 m ) of ots44 with sinfoni at the vlt at a medium spectral resolution ( r=@xmath11/@xmath12 , ) .
we discovered a strong , broad , and variable paschen@xmath1 ( pa@xmath1 ) emission line of ots44 in these spectra , as shown in fig.[fig : pabeta ] ( left , middle ) .
furthermore , a prominent h@xmath13 emission line is visible in the optical spectrum of @xcite as shown in the right panel of fig.[fig : pabeta ]
. both of these hydrogen emission lines exhibit a broad profile with velocities of @xmath14200kms@xmath15 or more .
we determined the equivalent width ( ew ) of both lines .
the h@xmath13 line has a symmetrically shaped profile with an ew of -141 , demonstrating that ots44 is actively accreting .
the profile of the pa@xmath1 line is significantly variable between the two observing epochs separated by a few days ( ew of -7 and -4 ) .
we determined the mass accretion rate of ots44 based on the h@xmath13 line by assuming that the h@xmath13 emission is entirely formed by accretion processes .
we derived an h@xmath13 line luminosity log@xmath16 ( @xmath17 ) of -6.16 for ots44 and applied the empirical relation between the h@xmath13 line and accretion luminosity of fang et al .
( 2009 ) , which is based on log@xmath16 ( @xmath17 ) ranging between -1 and -6 .
we estimate an mass accretion rate of ots44 of 7.6@xmath18@xmath19 .
pa@xmath1 emission of ots44 in vlt / sinfoni spectra ( left , middle ) and h@xmath13 emission of ots44 in a magellani / imacs spectrum ( right ) .
the dashed lines are gaussian fits to the profiles . from .
, title="fig : " ]   pa@xmath1 emission of ots44 in vlt / sinfoni spectra ( left , middle ) and h@xmath13 emission of ots44 in a magellani / imacs spectrum ( right ) .
the dashed lines are gaussian fits to the profiles . from .
, title="fig : " ]   pa@xmath1 emission of ots44 in vlt / sinfoni spectra ( left , middle ) and h@xmath13 emission of ots44 in a magellani / imacs spectrum ( right ) .
the dashed lines are gaussian fits to the profiles . from .
, title="fig : " ]
we have discovered strong , broad , and variable pa@xmath1 emission of the young very low - mass substellar object ots44 ( m9.5 ) in vlt / sinfoni spectra , which is evidence for active accretion of a planetary mass object .
we determined the properties of the disk that surrounds ots44 through ` mc3d ` radiative transfer modeling of flux measurements from the optical to the far - ir including herschel data .
we found that ots44 has a highly flared disk ( @xmath61.3 ) with a mass of 3.25@xmath7 , i.e. about @xmath8 .
we also investigated the h@xmath13 line of ots44 in a magellani / imacs spectrum spectrum and found strong h@xmath13 emission with an ew of -141 indicative of active accretion .
ots44 will become a y dwarf in @xmath20 years .
for this illustration of the evolution of the effective temperature of a very low - mass substellar object we used a plot of @xcite . note that the model temperatures are hotter than the observationally determined temperatures , which are 1700k for ots44 , 1200k for psoj318.5 - 22 , and 250k for wise0855 - 0714 . ]
both the pa@xmath1 and h@xmath13 emission lines of ots44 have broad profiles with the wings extending to velocities of about @xmath14200kms@xmath15 .
the pa@xmath1 emission is significantly variable on timescales of a few days , indicating variability in accretion - related processes of ots44 .
we estimated the mass accretion rate of ots44 to 7.6@xmath18@xmath21 by using the h@xmath13 line .
a mass accretion rate based on the pa@xmath1 line gives a significantly higher value , and we speculate that part of the pa@xmath1 emission might come from other processes related to accretion , such as outflows .
furthermore , in the course of studying ots44 , we fitted a photospheric bt - settl model to its optical and near - ir sed and derived a lower effective temperature and higher extinction than was previously found @xcite .
we have presented the first detection of pa@xmath1 emission for a free - floating object below the deuterium - burning limit .
our analysis of pa@xmath1 and h@xmath13 emission of ots44 demonstrates that free - floating objects of a few jupiter masses can be active accretors .
ots44 can be seen as free - floating analog of recently detected accreting planetary mass companions that orbit stars ( e.g. , @xcite , @xcite ) and it plays a key role in the study of disk evolution and accretion physics in an extremely low - gravity and -temperature environment . furthermore , ots44 ( m9.5 ) is the lowest - mass object to date for which the disk mass is determined based on far - ir data .
also interesting in this context is the mm - detection of an m9 dwarf @xcite . our detections therefore extend the exploration of disks and accretion during the t  tauri phase down to the planetary mass regime . plotting the relative disk masses of stars and brown dwarfs including ots44 ( fig.[fig : mdisk ] )
shows that the ratio of the disk - to - central - mass of about 10@xmath22 found for objects between 0.03@xmath23 and 14@xmath23 is also valid for ots44 at a mass of about 0.01@xmath23 .
furthermore , the mass accretion rate of ots44 is consistent with a decreasing trend from stars of several solar masses to substellar objects down to 0.01@xmath23 ( fig.[fig : macc ] ) .
it is also obvious from this figure that ots44 has a relatively high mass accretion rate considering its small mass .
these observations show that the processes that accompany canonical star formation , disks and accretion , are present down to a central mass of a few jupiter masses .
this suggests that ots44 is formed like a star and that the increasing number of young free - floating planets and ultra - cool t and y field dwarfs are the low - mass extension of the stellar population .
figure[fig : teffevol ] illustrates how a very young free - floating planetary mass object of about 10 jupiter masses , such as ots44 , cools down while aging and becomes an l dwarf at about 10myr , similar to psoj318.5 - 22 , and a y dwarf at @xmath241gyr , similar to wise0855 - 0714 .
this illustration uses a plot of the evolution of the effective temperature by @xcite .
while this might not be the most sophisticated evolutionary model to date and discrepancies with observations can be seen in the temperature ( see caption of fig.[fig : teffevol ] ) , it nevertheless shows very nicely the general picture of a substellar object shifting through spectral classes during its evolution .
we show that the coolest known object that is probably formed in a star - like mode is a free - floating planet . 
 we discovered recently that the free - floating planetary mass object ots44 ( m9.5 , @xmath012 jupiter masses , age @xmath02myr ) has significant accretion and a substantial disk . 
 this demonstrates that the processes that characterize the canonical star - like mode of formation apply to isolated objects down to a few jupiter masses . 
 we detected in vlt / sinfoni spectra that ots44 has strong , broad , and variable paschen @xmath1 emission . 
 this is the first evidence for active accretion of a free - floating planet . 
 the object allows us to study accretion and disk physics at the extreme and can be seen as free - floating analog of accreting planets that orbit stars . 
 our analysis of ots44 shows that the mass - accretion rate decreases continuously from stars of several solar masses down to free - floating planets . 
 we determined , furthermore , the disk mass ( 10 earth masses ) and further disk properties of ots44 through modeling its sed including herschel far - ir data . 
 we find that objects between 14 and 0.01 solar masses have the same ratio of the disk - to - central - mass of about 1% . 
 our results suggest that ots44 is formed like a star and that the increasing number of young free - floating planets and ultra - cool t and y field dwarfs are the low - mass extension of the stellar population .