Download Deep Infrared Images of Star-Forming - University of Missouri

Deep Infrared Images of Star-Forming
Cores in the Rho Oph Cloud
Dale Downs
Department of Physics and Astronomy
University of Missouri – St. Louis
Advisor: Dr. Bruce Wilking
Abstract
Observations were made and near-infrared photometry was performed in Cores A, E,
and E1 of the Ophiuchus Cloud. Photometry of objects from these cores was plotted
on infrared color-color and color-magnitude diagrams. Possible brown dwarfs were
selected from the completed data and plotted along with confirmed brown dwarfs on
another set of diagrams. The two goals of this research project were to identify new
candidate brown dwarfs and complete missing Hubble Space Telescope infrared
imagery in the Rho Oph Cloud.
Background
Objects in the sky emit several forms of radiation, including infrared and visible light.
Visible light was the main source of astronomical research until the mid 1940’s.
Astronomers could use telescopes to collect light from objects in space. Objects with
temperatures between 3,000 and 10,000 K emit most of their energy in visible light. Infrared
light is heat radiation. Infrared imaging allows lower temperature objects, more distant
bodies, and objects obscured by dust to be seen.
Infrared imaging made it possible to see into the Rho Ophiuchi Cloud. The Ophiuchus
molecular cloud region is one of the nearest star formation regions to Earth. It is estimated to
be 460 light years, or 140 parsecs, away. The cloud complex consists of a number of dark
clouds and when measured in the CO molecule is approximately 104 M (de Geus, Bronfman,
& Thaddeus 1989). At the center of cloud are several cold, dense cores with active star
formation. Star formation in Rho Oph is likely triggered by the compression of the cloud by
ionization fronts, supernovae explosions, and strong stellar winds from nearby B-type stars
(de Geus 1992). Stellar winds can then compress the medium surrounding the young stars
and produce shock-excited Herbig-Haro (HH) objects. Many of the stellar outflows may
carry enough momentum and kinetic energy to induce smaller cloud cores to collapse. There
are more than a 100 young stars concentrated in this area.
When there is insufficient gas available to form a star in a collapsing molecular cloud, the
body will not have a large enough mass or high enough temperature to sustain hydrogen
fusion. What results is called a brown dwarf. They are often referred to as “failed stars.” A
brown dwarf must have a mass less than 0.08 M, which is the critical mass required for
hydrogen fusion. Brown dwarfs usually have a mass between 12 MJupiter and 75 MJupiter. Its
core temperature must be less than 3 million degrees, again a critical value for nuclear
reactions. The surface temperature is estimated to be less than 2000 K. This can cool as the
brown dwarf ages. It is possible that nuclear reactions take place in the beginning of its life
but they do not last long. Brown dwarfs, as a result of their low surface temperature and
small size, are not very luminous. One goal of this project was to identify low-mass
members in the Rho Ophiuchi Cloud that could possibly be brown dwarfs.
The Hubble Space Telescope, or HST, was able to survey Ophiuchus (Allen et al. 2002), but
there was a problem with NICMOS, the Near Infrared Camera and Multi-Object
Spectrometer. NICMOS was said to be “the sharpest definition infrared instrument in
existence”1. Solid nitrogen was used to cool NICMOS so the heat from the camera would
not distort the infrared images. But the thermos flask that housed the nitrogen warped and
the nitrogen evaporated. Without the nitrogen coolant, NICMOS was unable to function
properly. HST was unable to observe complete infrared radiation in the 2.2 micron K-Band.
Thus, the other goal of this project was to collect information for the region in the K-Band.
Observations
Near-infrared photometry at J (1.25 m), H (1.6 m), and Ks (2.1 m) were obtained using
the University of Florida’s 20482 HgCdTe FLAMINGOS 1 camera. The observations were
made at the 6.5-m MMT telescope at Mt. Hopkins, Arizona on June 15th and 16th, 2001. The
scale was 0.166 pixel-1 resulting in a 5.7 x 5.7 field of view.
Three fields were observed: one in the Rho Oph A molecular core and two in the Rho Oph E
core. A set of observations included 5 dithers with integration times of 30 sec used for all
three filters. Every third set of observations was an “off” field within the cloud that is devoid
of bright objects and observed for purposes of sky-subtraction and flat-fielding.
Individual frames were sky-subtracted using a median-combined image of the off field
constructed from frames observed close in time. Flat-fielding was accomplished by dividing
by a median-combined image of all the dark-subtracted off-field images in that filter. After
adding a dc value representative of the sky background, bad pixels were masked, and the set
of 5 dithers were shifted and combined into a single mosaic which covered an area of
approximately 6.8 x 6.8.
Positions were determined using the astrometry program ASTROM and referenced to the
positions of 18 unsaturated sources in common with the survey of Barsony et al. (1997)
which were referenced to HST guide stars. The 1 errors in the positions are better than 0.7
relative to the BKLT reference frame and 1.4 relative to the HST guide stars.
Aperture photometry was performed on each mosaic using a 2.6 pixel aperture which is
approximately 2.5 times the FWHM of the stellar profile. Zero points for the photometry
were computed through comparison with the 2MASS database.
1
ESA, European Space Agency. NICMOS Returns!.
http://www.spacetelescope.org/about/history/sm3b_nicmos_returns.html
Results
We extracted 130 sources in Core A. The completeness limits were estimated from
histograms of the number of sources as a function of magnitude and were 18, 18.5, and 16.5
magnitudes at J, H, and Ks, respectively. We extracted 41 sources in Core E with
completeness limits of 19, 17.7, and 16 mag. We extracted 138 sources in Core E1 with
completeness limits of 19.5, 18.7, 17.5 mag. There are 7 sources in common between E and
E1. There are 61 sources that appear in the Allen et al. (2002) HST paper. There are 12
sources in common with the Keck NIRC spectroscopy performed by Meyer and Wilking
(2005, unpublished data). In particular is CRBR 23, which we classify as L1 and is also an
x-ray source.
Color-color Diagram:
The color-color diagram makes a comparison of the H-K and J-H color indices. The
resulting plot allows one to approximate the amount of dust in front of a star and surrounding
the star (its evolutionary phase). The objects from the cores A, E, and E1 were plotted in this
color-color diagram. Objects in field A are marked on the graph by the X symbol. The E1
core is represented by the ■ symbol. Field E is characterized by the ▲ symbol. Each object
was labeled with consideration for its one-sigma photometric errors. The bars surrounding
each object give a range that the star might fall in, with error in mind. The solid line figure at
the bottom of the graph indicates the locus of points for stars on the main sequence and in the
red giant phase. The upper solid line represents the red giants; and the bottom curved line
represents the main sequence stars. The dashed lines, or reddening lines, carry this
representation throughout the graph. If objects on this graph are reddened, they are moved
above the main sequence by a field of dust in front of them. The length of the arrow line
represents 10 magnitudes of visual extinction by dust. The amount of dust in front of a star
can be judged by the number of lengths away from the main sequence it falls. The CTTS,
Classical T Tauri Star, Locus line is displayed as the lowest of the three dashed lines; it is
also angled at the bottom. Stars on this line are possible young stars with a circumstellar
disk. Young stars are shifted towards this line because of emission by warm dust in their
inner disks which makes them brighter in the K band. Objects to the right of the CTTS locus
are brighter in the K band than expected for just a disk. They could have a disk and an
envelope of dust around them.
Color-Magnitude Diagram:
With a color-magnitude diagram, one can either assume an age and predict the mass or
assume a mass and predict the age. This color-magnitude diagram assumes an age so that we
can get an estimate of the mass. Mass is marked on the reddening lines in solar masses (M).
The solid lines are determined from the models of D’Antona and Mazzitelli (1997). The
dashed reddening lines, with a vector of Av=10 magnitudes, were configured from the
extinction law derived by Cohen et al. (1981). The completeness limit shown on the graph
uses a median of the 3 fields: K=16.5 and H=18.5. The symbols on this graph represent the
same fields they did in the color-color diagram.
Brown Dwarf Color-Color Diagram:
This graph is a color-color diagram just like the first except only confirmed brown dwarfs
and possible brown dwarfs are plotted on it. The X figure represents the stars in our field
that have been identified as brown dwarfs by examining their spectra. The ▲ figure plots the
positions of the brown dwarf candidates. Only two of our candidates were observed through
the J-Band and were able to be plotted on this graph. The rest were not bright enough to be
recorded at the J-Band. Objects to the right of the middle dashed line (the lower main
sequence reddening line) are in a good place to be classified as brown dwarfs but their
positions could vary due to scattered light.
Brown Dwarf Color-Magnitude Diagram:
Brown dwarfs are displayed on this graph as well. They are represented in the same manner
as the previous diagram. Brown dwarfs should fall below 0.08 solar masses on a colormagnitude graph. It is harder to determine whether objects below 0.017 solar masses are
older brown dwarfs or background stars. The two X’s below this line are confirmed not to be
background stars. Our candidates below this line are hard to classify. The candidate just
above the 0.04 solar mass line is GY 30. It is known to be a young object with a stellar
outflow and can be classified as a young brown dwarf.
Summary
Completing the first goal of this project, we were able to identify new brown dwarf
candidates. We compared the infrared colors of these candidates with those of confirmed
brown dwarfs identified through spectroscopy in an effort to predict the likelihood that the
candidate is a brown dwarf. Color-color and color-magnitude diagrams were created from
the infrared photometry of the region. If an object had a relatively low mass, appeared with a
low luminosity on the color-color and color-magnitude diagram, and gives off x-ray
radiation, it is very likely to be a newly formed brown dwarf and not a background star. The
work on this project was also able to supply missing data for the Hubble Space Telescope
infrared imagery of the Rho Oph Cloud.
References
Allen, L., Myers, P., Di Francesco, J., Mathieu, R., Chen, H., and Young, E. 2002,
Astrophysical Journal, 566, 993
Barsony, M., Kenyon, S. J., Lada, E. A., & Teuben, P. J., et al. 1997, Astrophysical Journal
Supplement, 112, 109
Cohen, J. G., Frogel, J. A., Persson, S. E., & Elias, J. H. 1981, Astrophysical Journal, 249,
481
D’Antona, F., & Mazzitelli, I. 1997, Mem. Soc. Aston. Italiana, 68, 807
de Geus, E., Bronfman, L., and Thaddeus, P. 1990, Astronomy & Astrophysics, 231, 137
de Geus, E. 1992, Astronomy & Astrophysics, 262, 258