Andrew John Monson

A radial velocity survey of the Cyg OB2 association

We conducted a radial velocity survey of the Cyg OB2 association over a 6 yr (1999-2005) time interval to search for massive close binaries. During this time we obtained 1139 spectra on 146 OB stars to measure mean systemic radial velocities and radial velocity variations. We spectroscopically identify 73 new OB stars for the first time, the majority of which are likely to be association members. Spectroscopic evidence is also presented for a B3 Iae classification and temperature class variation (B3-B8) on the order of 1 yr for Cyg OB2 No. 12. Calculations of the initial mass function with the current spectroscopic sample yield Γ = -2.2 ± 0.1. Of the 120 stars with the most reliable data, 36 are probable and 9 are possible single-lined spectroscopic binaries. We also identify three new and eight candidate double-lined spectroscopic binaries. These data imply a lower limit on the massive binary fraction of 30%-42%. The calculated velocity dispersion for Cyg OB2 is 2.44 ± 0.07 km s-1, which is typical of open clusters. No runaway OB stars were found.

The frequency of mid-infrared excess sources in galactic surveys

We have identified 230 Tycho-2 Spectral Catalog stars that exhibit 8 μm mid-IR extraphotospheric excesses in the MSX and Spitzer GLIMPSE surveys. Of these, 183 are either OB stars earlier than B8 in which the excess plausibly arises from a thermal bremsstrahlung component or evolved stars in which the excess may be explained by an atmospheric dust component. The remaining 47 stars have spectral classifications B8 or later and appear to be main-sequence or late pre-main-sequence objects harboring circumstellar disks. Six of the 47 stars exhibit multiple signatures characteristic of pre-main-sequence circumstellar disks, including emission lines, near-IR K-band excesses, and X-ray emission. Approximately one-third of the remaining 41 sources have emission lines suggesting relative youth. We modeled the excesses in 26 stars having two or more measurements in excess of the expected photospheres as single-component blackbodies. We determine probable disk temperatures and fractional IR luminosities in the range 191 K < T < 787 K and 3.9 × 10-4 < LIR/L* < 2.7 × 10-1. The majority of our modeled sample (14 stars) have 10-3 < LIR/L* < 10-2 and are consistent with either transition disks or massive debris disks. These objects have fractional IR luminosities and temperatures between those of β Pic-type debris disk systems (LIR/L* ≤ 10-3) and Class II pre-main-sequence systems (LIR/L* 10-1). We estimate a lower limit on the fraction of Tycho-2 Spectral Catalog main-sequence stars having mid-IR, but not near-IR, excesses to be 1.0% ± 0.3%.

The Carnegie-Chicago Hubble Program. II. The Distance to IC 1613: The Tip of the Red Giant Branch and RR Lyrae Period–luminosity Relations

IC 1613 is an isolated dwarf galaxy within the Local Group. Low foreground and internal extinction, low metallicity, and low crowding make it an invaluable testbed for the calibration of the local distance ladder. We present new, high-fidelity distance estimates to IC 1613 via its Tip of the Red Giant Branch (TRGB) and its RR Lyrae (RRL) variables as part of the Carnegie-Chicago Hubble Program, which seeks an alternate local route to \ho using Population II stars. We have measured a TRGB magnitude I=20.35+/-0.01 (statistical)+/-0.01 (systematic) using wide-field observations obtained from the IMACS camera on the Magellan-Baade telescope. We have further constructed optical and near-infrared RRL light curves using archival BI- and new H- band observations from the ACS/WFC and WFC3/IR instruments aboard the Hubble Space Telescope (HST). In advance of future Gaia data releases, we set provisional values for the TRGB luminosity via the Large Magellanic Cloud and Galactic RRL zero-points via HST parallaxes. We find corresponding true distance moduli \mu(TRGB)=24.30+/-0.03 (statistical)+/-0.05 (systematic) and \mu(RRL)=24.28+/-0.04 (statistical+systematic). We compare our results to a body of recent publications on IC 1613 and find no statistically significant difference between the distances derived from stars of Population I and II.

Erratum: “A Radial Velocity Survey of the Cygnus OB2 Association” (ApJ, 664, 1102 [2007])

The velocity dispersion of Cygnus OB2 was incorrectly quoted in the original manuscript as V 1⁄4 2:44 0:07 km s 1 for Vavg (the weighted average heliocentric velocities), and V 1⁄4 3:41 0:11 km s 1 for Vmid (the averages of the largest and smallest heliocentric velocities). The FWHM values for the Gaussian fits in Figure 23 are also incorrectly quoted as 5:70 0:17 km s 1 and 8:01 0:26 km s 1 for Vavg and Vmid, respectively. The correct one-dimensional velocity dispersions and corresponding FWHM values, as indicated by the Gaussian fits to the data in Figure 23, are V 1⁄4 5:74 0:15 km s 1 (Vavg), FWHM 1⁄4 13:50 0:36 km s 1 (Vavg), V 1⁄4 8:03 0:26 km s 1 (Vmid), and FWHM 1⁄4 18:87 0:60 km s 1 (Vmid). These values are larger than typical OB associations and open clusters (1Y2.5 km s 1; Bate & Bonnell, MNRAS, 356, 1201 [2005]). The larger dispersion estimates may be attributed to the uncertain systemic velocities of binaries (both identified and unidentified), owing to the limited orbital phase information from the current data set. The Astrophysical Journal, 681:735, 2008 July 1

NIIS: a wide-field, near-IR imaging spectrograph

Near-IR astronomical instrumentation is essential for the study of the distant universe. The finite speed of light enables astronomers to investigate the evolution of the universe by comparing the properties of galaxies far away to those found nearby. However, the ongoing expansion of the universe means that light emitted by stars and gas at optical wavelengths will be stretched or ‘redshifted’ to IR wavelengths by the time it reaches Earth. IR instruments also allow astronomers to characterize regions of star formation that are obscured at optical wavelengths by the gas and dust from which they form. The development of large-format, near-IR arrays has enabled the design and construction of a new generation of IR instruments for astronomical research.1 These arrays are now found in both the largest ground-based and space-based telescopes, as well as smaller instruments. They are often used in wide-field survey programs and in the study of time-variable, synoptic research programs. Some rare and transient events like gamma ray bursts have relatively poor positional determinations and fade rapidly. It is essential that follow-up imaging at optical and IR wavelengths be obtained as rapidly as possible so that these objects can be more precisely localized. To accomplish this, we are developing the Near-IR Imaging Spectrograph (NIIS). The wide field of view of NIIS and its IR imaging capability will enable rapid characterization of the most distant and highly redshifted examples of these objects. NIIS was designed for wide-field imaging surveys on mid-sized astronomical telescopes and was recently commissioned at the Apache Point Observatory’s 3.5m telescope near Alamogordo, New Mexico.2 The optical design of NIIS features a traditional corrector C collimator C camera configuration that allows the images from the telescope that are limited by atmospheric turbulence to be reduced to better match the pixels of the 2048 2048 Hawaii2 detector and produce the widest practical field of view Figure 1. A cross-section of the Near-IR Imaging Spectrograph (NIIS). Light from the telescope enters the vacuum cryostat from the left through a window and passes through three lens stacks. The cold pupil stop and filter wheel (yellow) are located in the collimated beam after which the camera elements re-image the light onto the detector. The internal lens spacers of the collimator lens stack are visible, whereas those of the camera and the internal thermal shielding are not shown, for clarity.