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

Abstract

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 M J , 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 ; 10 6 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.

The theory of globulettes: candidate precursors of brown dwarfs and free-floating planets in H ii regions

“…In this latter case the globule has a more random motion determined by the initial turbulent velocity field. Gahm et al (2007) showed that the expected photoevaporation timescale for globulettes is around 3.8 Myr; this value is consistent with the fact that images of globulettes show a mixture of spherical and cometary morphologies. Grenman & Gahm (2014) found that some globulettes in Carina were particularly small and dense, leading the authors to conclude that perhaps they were at a later stage in globulette evolution, having had more time to be compressed or for lower density outer layers to be dispersed.…”

“…by Gahm et al 2007). The picture we have is that there is an expanding H ii region, bounded by a dense shell, from which pillars protrude into the ionised gas.…”

“…The green dashed-dotted line (with circled markers) shows the same result for Carina. Brown triangles are globulettes in the Rosette, red crosses are globulettes in the Carina nebula (Grenman & Gahm 2014), blue triangles in IC 1805 and black circles in NGC 7822 (Gahm et al 2007). …”

“…Doing so over a range of masses yields the theoretical massradius relation for globulettes. In Figure 2 the solid black line shows our theoretical mass-radius relation for an external pressure Pext = nkBT , where the electron density n in the H ii region is 10 cm −3 , and the average temperature of the ionized gas is 5800 K (that estimated for the Rosette by Celnik 1985 Gahm et al 2007). These pressure confined Bonnor-Ebert spheres are not very stratified, having a broad interior region of slowly varying density until close to the globulette boundary (c.f.…”

“…Gahm et al (2007) estimated globulette masses using extinction maps derived from deep narrowband Hα images, collected with the 2.6 m Nordic Optical Telescope (Gahm et al 2007). For Carina, Grenman & Gahm (2014) used Hα data from the Hubble Space Telescope archive.…”