Alcalá et al., "Multi-wavelength Observations of
the Star-forming Region in L1616." 2004, A&A, 516, 677 (PDF)
Alcalá et al., "X-ray and Optical Observations of
NGC 1788." 2003, ASPC, 287, 140 (conference
Cooksey, Hoard, & Wachter, "Star Formation in the Isolated
Molecular Cloud NGC 1788 (LDN 1616)," poster,
199th Meeting of the American Astronomical Society,
Washington, D.C. (January 2002)
Day et al., "Light and Color Curves of Six Field RR Lyrae
Variable Stars." 2002, PASP,
114, 645 (PDF)∗
"Star Formation in the Isolated Molecular Cloud NGC1788
(LDN1616)," 2001, (in-house presentation;
∗(Supplementary observing on the Schmidt 1-m telescope for Andrew Layden)
In the standard model of star formation, UV radiation from OB stars or supernovae
shockwaves compresses the cold material in giant molecular clouds. Dense cores
collapse due to self-gravity and accrete material until there is enough pressure to
ignite nucleosynthesis, marking the beginning of the proto-star phase. This model
successfully describes large conglomerations of stars. However, the spatial distribution
of stars in the Universe cannot be fully explained if stars only form in large clusters.
Thus, effective star formation in isolated molecular clouds, far from the massive complexes
but most likely still induced by them, offers an explanation for the observed distribution
of stars. The region around NGC 1788, approximately 50 pc west of the Orion OB association,
is such an isolated cloud.
We conducted a multi-wavelength survey of the NGC 1788 field to
characterize how star formation proceeds in isolated molecular clouds. We present
near-infrared color-color and color-magnitude diagrams for stars in this region, including
correlations between the X-ray, optical, and near-IR sources. In addition we have obtained
optical spectra of the X-ray source counterparts. The majority show Li l6708 absorption,
confirming their status as young objects associated with NGC 1788.
Figure 1: UBVRI False-color Image
NGC 1788 is a reflection nebula located at (5h6m48s,
-3°23′0″). The illuminating star is most likely HD293815, a B9V star (see Figure 2).
LDN 1616 is the dusty nebula encompassing NGC 1788. LDN 1616 (the "head") also blends with
LDN 1615 (the "tail"), forming a cometary shape
extending to the southwest (away from the
Orion OB association) and probably caused by UV radiation pressure from the Orion OB association.
Many estimates of the distance to NGC 1788 exist in the literature, ranging from 360 to 460 pc
(Ogura 1998, Racine 1968).
This is a UBVRI false color image of the central region of NGC 1788, constructed from images
we obtained on the CTIO 0.9-m telescope. The image is 13.5′ x 13.5′.
Figure 2: Near-infrared and Optical Images of NGC 1788
This figure shows near-infrared (left) and optical (right) images of the central
region of NGC 1788 on the same spatial scales (5.8′ x 6.7′). The near-IR image
is in the K band from 2MASS; the optical is in the V band from the CTIO 0.9-m telescope.
A total of 22 ROSAT X-ray sources lie within 30′ of the center of NGC 1788. X-ray
sources with near-IR and optical counterparts identified by positional matches are
indicated in the figures (and labeled with the arbitrary X-ray source number from our
target list (see Figure 4). Two possible counterparts for both X-ray sources 11 and
12 are blended in the near-IR and optical images; we refer to these as 11a and 11b,
12a and 12b. The extended region labeled Blob 12 (in the optical) is resolved into
several stellar sources in the near-IR.
Figure 3: Near-infrared and Optical Images of NGC 1788
We extracted the objects located within 15′ of the center of NGC 1788 from the 2MASS
Second Incremental Data Release Point Source Catalogue (black points). 2MASS sources
with ROSAT X-ray counterparts are indicated in red and labeled with arbitrary numbers
from our target list (also see Figure 4). The main sequence (green) and giant branch
(blue) are shown, using data from Cox (2001) transformed to the 2MASS photometric
system via the relations in Carpenter (2001). The 2MASS Ks absolute magnitudes in the
color-magnitude diagram were calculated assuming a distance of 400 pc without accounting
for interstellar or intrinsic reddening. An interstellar (de)reddening vector is shown
in the color-color diagram, but we have not yet characterized the potentially
differential intrinsic reddening of NGC 1788. Also, object 11a is outside the boundaries
of the color-color diagram, at (J-H)=2.256 and (H-K)=0.484. The color-magnitude diagram
and optical spectra (see Figure 4) indicate that NGC 1788 contains primarily young, low
mass stars. The majority of the ROSAT X-ray sources indicated in Figure 2 appear to be
classical T-Tauri stars from their optical spectra (see Figure 4) and appear to be
approaching the main sequence in the color-magnitude diagram.
Figure 4: Optical Spectra of Counterparts to X-ray Sources in NGC 1788
We obtained medium resolution (5Å) optical spectra of the counterparts to the
ROSAT X-ray sources in NGC 1788 using the R-C Spectrograph on the CTIO 1.5-m telescope.
The spectra correspond to the same-numbered locations indicated in Figure 2. Sources
7, 9, 11a, 12a & b, 15, 17, and 21 show lithium absorption,
indicating that they are
young stars. In fact, sources 15 and 17 appear to be classical T-Tauri stars. Blob
12 is the bright, extended region indicated in Figure 2 (right).
In general, we are examining NGC 1788 at multiple wavelengths in order to determine
properties of young stars in an isolated star-forming region across a broad range of
the electromagnetic spectrum. We have correlated ROSAT X-ray sources with their
near-infrared and optical counterparts and magnitudes. We have most likely identified
young stars in NGC 1788 based on their locations on color-color and color-magnitude
diagrams and their optical spectra and lithium absorption. We also have radio and
mid-IR data of NGC 1788, which will be incorporated into our study.