Table of contents

We have observed a 3° × 3° area centered on the M81/M82 group of galaxies using the Robert C. Byrd Green Bank Telescope in a search for analogs to the high-velocity clouds of neutral hydrogen found around our Galaxy. The velocity range from −605 to −85 km s⁻¹ and from 25 to 1970 km s⁻¹ was searched for H i clouds. Over the inner 2° × 2° the 7σ detection threshold was 9.6 × 10⁵ M_☉. We detect five previously unknown H i clouds associated with the group, as well as numerous associated filamentary H i structures, all lying in the range −105 ⩽ V_helio ⩽ +280 km s⁻¹. From the small angular distance of the clouds to group members, and the small velocity difference between group members and clouds, we conclude that the clouds are most likely relics of ongoing interactions between galaxies in the group.

We have obtained single-phase near-infrared magnitudes in the J and K bands for a sample of 78 RR Lyrae stars in the Sculptor dSph galaxy. Applying different theoretical and empirical calibrations of the period–luminosity–metallicity relation for RR Lyrae stars in the infrared, we find consistent results and obtain a true, reddening-corrected distance modulus of 19.67 ± 0.02 (statistical) ± 0.12 (systematic) mag for Sculptor from our data. This distance value is consistent with the value of 19.68 ± 0.08 mag which we obtain from earlier V-band data of RR Lyrae stars in Sculptor, and the V magnitude metallicity calibration of Sandage. It is also in very good agreement with the results obtained by Rizzi based on the tip of the red giant branch (TRGB, 19.64 ± 0.08 mag) and horizontal branch (HB, 19.66 ± 0.15 mag).

We present the largest and most detailed survey to date of the stellar populations in the outskirts of M31 based on the homogeneous analysis of 14 deep Hubble Space Telescope Advanced Camera for Surveys (HST/ACS) pointings spanning the range 11.5 kpc ≲ R_proj ≲ 45 kpc. Many of these pointings sample coherent substructure discovered in the course of the Isaac Newton Telescope Wide Field Camera (INT/WFC) imaging survey of M31 while others sample more diffuse structure in the extended disk. We conduct a quantitative comparison of the resolved stellar populations in these fields and identify several striking trends. The color–magnitude diagrams (CMDs), which reach ≳3 mag below the red clump with high completeness, can be classified into two main categories based on their morphologies. "Stream-like" fields, so named for their similarity to the CMD of the giant stellar stream, are characterized by a red clump that slants blueward at fainter magnitudes and an extended horizontal branch. They show no evidence for young populations. On the other hand, "disk-like" fields exhibit rounder red clumps with significant luminosity width, lack an obvious horizontal branch, and show evidence for recent star formation (∼0.25–2 Gyr ago). We compare the spatial and line-of-sight distribution of stream-like fields with a recent simulation of the giant stream progenitor orbit and find an excellent agreement. These fields are found across much of the inner halo of M31, and attest to the high degree of pollution caused by this event. Disk-like material resides in the extended disk structure of M31 and is detected here up to R_proj ∼ 44 kpc; the uniform populations in these fields, including the ubiquitous presence of young populations, and the strong rotation reported elsewhere are most consistent with a scenario in which this structure has formed through heating and disruption of the existing thin disk, perhaps due to the impact of the giant stream progenitor. Our comparative analysis sheds new light on the likely composition of two of the ultra-deep pointings formerly presented as pure outer disk and pure halo in the literature.

We present a 3D kinematical analysis of stars located in Kapteyn Selected Area 71 (l = 167.1°,  b = −34.7°), where previously a stellar excess was found by Dinescu et al. in (2002). Previous findings indicated that the stellar excess has a cold kinematical signature as inferred from proper motions, and was initially associated with debris from the Sagittarius dwarf galaxy (Sgr)—namely the southern trailing tail. We have obtained radial velocities using the Hydra multi-object spectrograph on the WIYN 3.5 m telescope. Results for 183 proper-motion-selected stars indicate that the dominant population in this stellar excess is not debris from Sgr, but rather a population that kinematically belongs to the ring-like stream that is now known as the Monoceros (Mon) stream. The orbit determined for this population agrees very well with the predictions for the Mon stream from Peñarrubia et al. in (2005). The radial-velocity dispersion of this population is between 20 and 30 km s⁻¹, lower than that of the Galactic field. Also, the shape of the radial-velocity distribution shows a sharp cut-off on one side, which is more in line with model predictions of the disruption of a satellite rather than with the distribution of the Galactic field. Despite the fact that we now believe most of the stars in the stellar excess to be a part of Mon, about ten stars in this stellar excess have highly negative radial velocities, which is a clear indication of their membership to the Sgr trailing tidal tail.

We present the results of a survey of nearby, young M stars for wide low-mass companions with the Infrared Array Camera (IRAC) on the Spitzer Space Telescope. We observed 40 young M dwarfs within 20 pc of the Sun, selected through X-ray emission criteria. A total of ten candidate companions were found with IRAC colors consistent with T dwarfs. Extensive ground-based NIR follow-up observations rejected all these candidates. Two additional candidates were discovered via common proper motion measurements, one of which was rejected as a background object and the other is a bona fide companion to GJ 2060, a member of the AB Doradus moving group.

We do an initial reconnaissance of asymmetries of spectral lines in metal-poor field stars using high-resolution observations of four red horizontal-branch and 11 red giant branch stars taken with the coude spectrograph at the Canada–France–Hawaii Telescope. We find that (1) the shapes of the line bisectors for metal-poor stars hotter than 4100 K mimic the well-known C shape of bisectors for solar-metallicity stars on the cool side of the granulation boundary while (2) metal-poor stars cooler than 4100 K, or higher up the red giant branch than M_V = −1.5, show bisectors with a reversed-C shape, similar to those for solar-metallicity stars on the hot side of the granulation boundary and similar to the reversed-C shape found in a previous study for the M-type supergiant Betelgeuse. The well-documented radial-velocity jitter of high-luminosity stars and the line bisector characteristics vary in concert up the red giant branch; both phenomena are probably signatures of large convection cells.

Deep Hα observations of the Sculptor Group galaxy NGC 7793 were obtained on the ESO 3.60 m and the Marseille 36 cm telescopes at La Silla, Chile. Hα emission is detected all the way to the edge of the H i disk, making the H ii disk of NGC 7793 one of the largest ever observed in a quiet non-active galactic nucleus (AGN) late-type system. Even in the very outer parts, the H ii ionizing sources are probably mainly internal (massive stars in the disk) with an unlikely contribution from the extragalactic ionizing background. The Hα kinematics confirms what had already been seen with the H i observations: NGC 7793 has a truly declining rotation curve. However, the decline is not Keplerian and a dark halo is still needed to explain the rotation velocities in the outer parts.

New spectra of NGC 2992 from the Cerro Tololo Inter-American Observatory show that this nearby active galactic nucleus changed its type classification to a Seyfert 2 in 2006. It was originally classified as a Seyfert 1.9, and has been previously seen as a Seyfert 1.5 with strong broad Hα emission. A comparison of the reddening and equivalent hydrogen column density derived for the narrow-line region from these new data with those previously calculated for different regions closer to the nucleus shows them to be very similar, and suggests that these different regions are all being absorbed by the same opacity source, a large 100 pc scale dust lane running across the nucleus. However, obscuration by dust in this lane is probably not responsible for classification changes which occur in only a few years. It is more likely that NGC 2992's observed variations are due to a highly variable ionizing continuum. We therefore conclude that, although NGC 2992 was originally identified as a Seyfert 1.9, this was not because of an oblique viewing angle through the atmosphere of a central dusty torus, but because its active nucleus was identified when it was in a low-continuum state.

The ACS Survey of Globular Clusters has used Hubble Space Telescope's Wide-Field Channel to obtain uniform imaging of 65 of the nearest globular clusters to provide an extensive homogeneous data set for a broad range of scientific investigations. The survey goals required not only a uniform observing strategy, but also a uniform reduction strategy. To this end, we designed a sophisticated software program to process the cluster data in an automated way. The program identifies stars simultaneously in the multiple dithered exposures for each cluster and measures them using the best available point-spread function models. We describe here in detail the program's rationale, algorithms, and output. The routine was also designed to perform artificial-star tests, and we ran a standard set of ∼10⁵ tests for each cluster in the survey. The catalog described here will be exploited in a number of upcoming papers and will eventually be made available to the public via the World Wide Web.

We present Very Large Array observations of H₂O and OH masers as well as radio continuum emission at 1.3 and 18 cm toward three sources previously cataloged as planetary nebulae (PNe) and in which single-dish detections of H₂O masers have been reported: IRAS 17443−2949, IRAS 17580−3111, and IRAS 18061−2505. Our goal was to unambiguously confirm their nature as water-maser-emitting PNe, a class of objects of which only two bona fide members were previously known. We detected and mapped H₂O maser emission toward all three sources, while OH maser emission is detected in IRAS 17443−2949 and IRAS 17580−3111 as well as in other two objects within the observed fields: IRAS 17442−2942 (unknown nature) and IRAS 17579−3121 (also cataloged as a possible PN). We found radio continuum emission associated only with IRAS 18061−2505. Our results confirm IRAS 18061−2505 as the third known case of a PN associated with H₂O maser emission. The rest of the studied sources are not likely to trace PNe. The three known water-maser-emitting PNe have clear bipolar morphologies, which suggests that water maser emission in these objects is related to non-spherical mass-loss episodes. We speculate that these bipolar water-maser-emitting PNe would have "water-fountain" asymptotic giant branch (AGB) and post-AGB stars as their precursors. A note of caution is given for other objects that have been classified as OHPNe (objects with both OH maser and radio continuum emission that could be extremely young PNe) based on single-dish observations, since interferometric data of both OH masers and continuum are necessary for a proper identification as members of this class.

Willman 1 is a small low-surface-brightness object identified in the Sloan Digital Sky Survey and tentatively classified as a very low luminosity dSph galaxy. Further study has supported this classification while hinting that it may be undergoing disruption by the Milky Way potential. In an effort to better constrain the nature of Willman 1, we present a comprehensive analysis of the brightest stars in a 0.6 deg² field centered on the overdensity. High-resolution Hobby-Eberly Terlescope (HET) spectra of two previously identified Willman 1 red giant branch (RGB) stars show that one is a metal-rich foreground dwarf while the other is a metal-poor giant. The one RGB star that we confirm as a member of Willman 1 has a low metallicity ([Fe/H] = −2.2) and a surprisingly low α-element abundance ([α/Fe] = −0.11). Washington+DDO51 photometry indicates that 2–5 of the seven brightest Willman 1 stars identified in previous studies are actually dwarf stars, including some of the more metal-rich stars that have been used to argue both for an abundance spread and a more metal-rich stellar population than galaxies of similar luminosity. The remaining stars are too blue or too faint for photometric classification. The Washington+DDO51 photometry identifies three potential RGB stars in the field but HET spectra show that they are background halo stars. Time series photometry identifies one apparent variable star in the field, but it is unlikely to be associated with Willman 1. Our wide-field survey indicates that over 0.6 deg², Willman 1 does not have a single RR Lyrae star, a single blue horizontal branch (BHB) star, or a single RGB star beyond its tidal radius. While our results confirm that Willman 1 is most likely a low-luminosity metal-poor dSph galaxy, the possibility remains that it is a tidally disrupted metal-poor globular cluster.

We investigate three embedded massive star-forming regions using the Near-Infrared Imager (NIRIM) camera on the 3.5 m WIYN telescope. We report J-, H-, and K'-band photometry in the clusters AFGL437, AFGL5180, and AFGL5142, and use these results to probe the stellar populations, extinction, and ages of the clusters. We find that all three clusters suffer significant extinction (A_K ∼ 1), have ages ⩽ 5 Myr, and are actively forming stars. The background-corrected percentages of cluster members identified as young stellar objects (YSOs) in AFGL437, AFGL5180, and AFGL5142 are 58% ± 15%, 75% ± 10%, and 78% ± 15%, respectively. The slopes of the K-band luminosity functions (KLFs) for these clusters are ∼0.3–0.4, similar to that in other young clusters. Both the percentage of YSOs and KLF slopes of these clusters indicate that their ages are <5 Myr. We investigate the consequences of incompleteness in areas of high background emission and stellar density, and find that it contributes significant uncertainty to the determination of cluster properties. We find that age determination using only near-IR (NIR) photometry has an uncertainty of ∼3 Myr.

AM CVn systems are a select group of ultracompact binaries with the shortest orbital periods of any known binary subclass; mass transfer is likely from a low-mass (partially-)degenerate secondary onto a white dwarf primary, driven by gravitational radiation. In the past few years, the Sloan Digital Sky Survey (SDSS) has provided five new AM CVns. Here we report on two further candidates selected from more recent SDSS data. SDSS J1208+3550 is similar to the earlier SDSS discoveries, recognized as an AM CVn via its distinctive spectrum which is dominated by helium emission. From the expanded SDSS Data Release 6 (DR6) spectroscopic area, we provide an updated surface density estimate for such AM CVns of order 10^−3.1–10^−2.5 deg⁻² for 15 < g < 20.5. In addition, we present another new candidate AM CVn, SDSS J2047+0008, which was discovered in the course of follow-up of SDSS-II supernova candidates. It shows nova-like outbursts in multi-epoch imaging data; in contrast to the other SDSS AM CVn discoveries, its (outburst) spectrum is dominated by helium absorption lines, reminiscent of KL Dra, and 2003aw. The variability selection of SDSS J2047+0008 from the 300 deg² of SDSS Stripe 82 presages further AM CVn discoveries in future deep, multicolor, and time-domain surveys such as the Large Synoptic Survey Telescope (LSST). The new additions bring the total SDSS yield to seven AM CVns thus far, a substantial contribution to this rare subclass, versus the dozen previously known.

We describe here the reduction methods that we developed to study the faintest red dwarfs and white dwarfs in an outer field of NGC 6397, which was observed by HST for 126 orbits in 2005. The particular challenge of this data set is that the faintest stars are not readily visible in individual exposures, so special care must be taken to combine the information in all the exposures in order to identify and measure them. Unfortunately, it is hard to find the faintest stars without also finding a large number of faint galaxies, so we developed specialized tools to distinguish between the point-like stars and the barely resolved galaxies. We found that artificial-star tests, while obviously necessary for completeness determination, can also play an important role in helping us optimize our finding and measuring algorithms. Although this paper focuses on this data set specifically, many of the techniques are new and might find application in other work, particularly when a large number of images are available for a single field.

We present N-body models to complement deep imaging of the metal-poor core-collapsed cluster NGC 6397 obtained with the Hubble Space Telescope. All simulations include stellar and binary evolution in step with the stellar dynamics and account for the tidal field of the Galaxy. We focus on the results of a simulation that began with 100, 000 objects (stars and binaries), 5% primordial binaries, and Population II metallicity. After 16 Gyr of evolution, the model cluster has about 20% of the stars remaining and has reached core collapse. We compare the color–magnitude diagrams of the model at this age for the central region and an outer region corresponding to the observed field of NGC 6397 (about 2–3 half-light radii from the cluster center). This demonstrates that the white dwarf (WD) population in the outer region has suffered little modification from dynamical processes—contamination of the luminosity function by binaries and WDs with non-standard evolution histories is minimal and should not significantly affect measurement of the cluster age. We also show that the binary fraction of main-sequence stars observed in the NGC 6397 field can be taken as representative of the primordial binary fraction of the cluster. For the mass function (MF) of the main-sequence stars, we find that although this has been altered significantly by dynamics over the cluster lifetime, especially in the central and outer regions, the position of the observed field is close to optimal for recovering the initial MF of the cluster stars (below the current turn-off mass). More generally we look at how the MF changes with radius in a dynamically evolved stellar cluster and suggest where the best radial position to observe the initial MF is for clusters of any age. We discuss computational constraints that restrict the N-body method to non-direct models of globular clusters currently, how this affects the interpretation of our results regarding NGC 6397, and future plans for models with increased realism.

We present the color–magnitude diagram (CMD) from deep Hubble Space Telescope imaging in the globular cluster NGC 6397. The Advanced Camera for Surveys (ACS) was used for 126 orbits to image a single field in two colors (F814W, F606W) 5' SE of the cluster center. The field observed overlaps that of archival WFPC2 data from 1994 and 1997 which were used to proper motion (PM) clean the data. Applying the PM corrections produces a remarkably clean CMD which reveals a number of features never seen before in a globular cluster CMD. In our field, the main-sequence stars appeared to terminate close to the location in the CMD of the hydrogen-burning limit predicted by two independent sets of stellar evolution models. The faintest observed main-sequence stars are about a magnitude fainter than the least luminous metal-poor field halo stars known, suggesting that the lowest-luminosity halo stars still await discovery. At the bright end the data extend beyond the main-sequence turnoff to well up the giant branch. A populous white dwarf cooling sequence is also seen in the cluster CMD. The most dramatic features of the cooling sequence are its turn to the blue at faint magnitudes as well as an apparent truncation near F814W = 28. The cluster luminosity and mass functions were derived, stretching from the turnoff down to the hydrogen-burning limit. It was well modeled with either a very flat power-law or a lognormal function. In order to interpret these fits more fully we compared them with similar functions in the cluster core and with a full N-body model of NGC 6397 finding satisfactory agreement between the model predictions and the data. This exercise demonstrates the important role and the effect that dynamics has played in altering the cluster initial mass function.

Using Hubble Space Telescope observations of the globular cluster NGC 6397, we constrain the cluster's binary fraction. The observations consist of two fields: the primary science field, a single Advanced Camera for Surveys (ACS) pointing centered approximately 5' from the cluster center; and the parallel field, a single WFPC2 field centered on the cluster center. Using the exquisite photometric precision of these observations, we determine the binary fraction in these regions of the cluster by examining stars lying off the main sequence. The binary fraction is constrained to be 0.012 ± 0.004 in the ACS field, and to be 0.051 ± 0.010 in the WFPC field. N-body simulations by Hurley et al. in 2007 suggest that the binary fraction remains nearly constant beyond the half-mass radius for the lifetime of the cluster. In the context of these simulations, our results suggest that NGC 6397 had a primordial binary fraction of only ∼1%.

We present the first detailed photometric and spectroscopic study of the white dwarfs (WDs) in the field of the ∼225 Myr old (log τ_cl = 8.35) open cluster NGC 1039 (M34) as part of the ongoing Lick–Arizona White Dwarf Survey. Using wide-field UBV imaging, we photometrically select 44 WD candidates in this field. We spectroscopically identify 19 of these objects as WDs; 17 are hydrogen-atmosphere DA WDs, one is a helium-atmosphere DB WD, and one is a cool DC WD that exhibits no detectable absorption lines. We find an effective temperature (T_eff) and surface gravity (log g) for each DA WD by fitting Balmer-line profiles from model atmospheres to the observed spectra. WD evolutionary models are then invoked to derive masses and cooling times for each DA WD. Of the 17 DAs, five are at the approximate distance modulus of the cluster. Another WD with a distance modulus 0.45 mag brighter than that of the cluster could be a double-degenerate binary cluster member, but is more likely to be a field WD. We place the five single cluster member WDs in the empirical initial–final mass relation and find that three of them lie very close to the previously derived linear relation; two have WD masses significantly below the relation. These outliers may have experienced some sort of enhanced mass loss or binary evolution; however, it is quite possible that these WDs are simply interlopers from the field WD population. Eight of the 17 DA WDs show significant Ca ii K absorption; comparison of the absorption strength with the WD distances suggests that the absorption is interstellar, though this cannot be confirmed with the current data.

Completion of the SUPERBLINK proper motion survey in the southern celestial hemisphere has turned up 170 new stars with proper motion 0.45'' yr⁻¹ < μ < 2.0'' yr⁻¹. This fourth and final installment completes the all-sky, data mining of the Digitized Sky Surveys for stars with large proper motions. The areas investigated in this final installment comprise 11,600 deg⁻² in the declination (decl.) range −30°< decl. <0° and in low Galactic latitude areas south of decl. = −30° which had not been covered in earlier data releases. Astrometric and photometric data are provided for the 170 new stars, along with finder charts. Most of the new discoveries are found in densely populated fields along the Milky Way, toward the Galactic bulge/center. The list of new discoveries includes four stars with proper motion μ > 1.0'' yr⁻¹. The total list of high proper motion stars recovered by SUPERBLINK in the southern sky now includes 2228 stars with proper motions 0.45'' yr⁻¹ < μ < 2.0'' yr⁻¹.

Using both the Very Large Array (VLA) at 7 mm wavelength, and the Australia Telescope Compact Array (ATCA) at 3 mm, we have searched for microwave emission from cool dust in the extrasolar planetary system Gliese 876 (Gl 876). Having detected no emission above our 3σ detection threshold of 135 μJy, we rule out any dust disk with either a mass greater than 0.0006 M_⊕ or less than ∼250 AU across. This result improves on previous detection aperture thresholds by an order of magnitude, and it has some implications for the dynamical modeling of the system. It also is consistent with the Greaves et al. hypothesis that relates the presence of a debris disk to close-in planets. Due to the dust-planetesimal relationship, our null result may also provide a constraint on the population or composition of the dust and small bodies around this nearby M dwarf.

We present the kinematics of a sample of bipolar planetary nebulae (PNs) which cover a wide range of observed morphologies and collimation degrees, from bipolar PNs (BPNs) with a marked equatorial ring and wide lobes to highly collimated objects. We use an empirical model in order to derive the expansion velocity, collimation degree, and inclination angle of the PN with respect to the plane of the sky. The equatorial expansion velocities measured in the objects in our sample are always in the low-to-medium range (3–16 km s⁻¹), while their polar expansion velocities range from low to very high (18–100 km s⁻¹). None of the objects in our sample, even those that show an extreme collimation degree, seem to be (kinematically) younger than ≃10³ yr. We compare our results with the state-of-the-art theoretical models for the formation of BPNs. We find good agreement between the observed expansion velocities and numerical models that use magnetic fields with stellar rotation as a collimation mechanism.

We have analyzed the radio light curves of PKS 1510-089 at 37 and 22 GHz from 1990 to 2005 taken from the database of the Metsähovi Radio Observatory, and find evidence of quasi-periodic outbursts. The light curves show great activity with very complicated non-sinusoidal variations. Using Jurkervich's method, the power spectrum method, and the discrete autocorrelation function to analyze these data, we have found two periods of p₁ = 0.92 ± 0.04 yr and p₂ = 1.82 ± 0.12 yr for the outbursts in PKS 1510-089. It is interesting to note that the results for two frequencies and three methods are almost the same and p₂ ≈ 2p₁. In addition, these results are in good agreement with the periodic deep flux minima of 1.84 ± 0.1 yr (half period ∼ 0.92 ± 0.03 yr) observed by us and other authors in the optical band in 2002, 2004, and 2005.

We draw attention to the singularly peculiar photometric properties of the 9.5-day Cepheid FN Aql. An initial study of its color–color plots shows an unusually wide trajectory in the (U − B)–(B − V) diagnostic plane. A closer investigation reveals that the photometric residuals around fits to the individual mean light curves are highly correlated, decreasing in amplitude as a function of increasing wavelength. That correlation and trend suggest time-variable dust extinction; however, the grain properties must be different from the general interstellar medium. FN Aql is an IRAS source and also appears in the proto–planetary nebulae candidate in the 1989 listing of Volk & Kwok. The source of the changing extinction is probably localized and intrinsic to the Cepheid itself, possibly a wind. Further photometric studies of this peculiar Cepheid are recommended.

We present a multi-wavelength study of embedded massive clusters in the nearby (3.9 Mpc) starburst galaxy NGC 4449 in an effort to uncover the earliest phases of massive cluster evolution. By combining high-resolution imaging from the radio to the ultraviolet, we reveal these clusters to be in the process of emerging from their gaseous and dusty birth cocoons. We use Very Large Array (VLA) observations at centimeter wavelengths to identify young clusters surrounded by ultra-dense H ii regions, detectable via their production of thermal free–free radio continuum. Ultraviolet, optical and infrared observations are obtained from the Hubble and Spitzer Space Telescope archives for comparison. We detect 39 compact radio sources toward NGC 4449 at 3.6 cm using the highest resolution (13) and sensitivity (∼12 μJy) VLA image of the galaxy to date. We reliably identify 13 thermal radio sources and derive their physical properties using both nebular emission from the H ii regions and spectral energy distribution fitting to the stellar continuum. These radio-detected clusters have ages ≲5 Myr and stellar masses of order 10⁴M_☉. The measured extinctions are quite low: 12 of the 13 thermal radio sources have A_V ≲ 1.5, while the most obscured source has A_V ≈ 4.3. By combining results from the nebular and stellar emission, we find an I-band excess that is anti-correlated with cluster age and an apparent mass–age correlation. Additionally, we find evidence that local processes such as supernovae and stellar winds likely play an important role in triggering the current bursts of star formation within NGC 4449.

We present the first results of a long-term program of a radial velocity (RV) study of Cepheid Polaris (F7 Ib), with the aim of finding the amplitude and period of its pulsations and the nature of its secondary periodicities. A total of 264 new precise RV measurements were obtained during 2004–2007 with the fiber-fed echelle spectrograph Bohyunsan Observatory Echelle Spectrograph (BOES) of the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We find a pulsational RV amplitude and period of Polaris for the three seasons 2005.183, 2006.360, and 2007.349 as 2K = 2.210 ± 0.048 km s⁻¹, 2K = 2.080 ± 0.042 km s⁻¹, and 2K = 2.406 ± 0.018 km s⁻¹ respectively, indicating that the pulsational amplitudes of Polaris that had decayed during the last century are now increasing rapidly. The pulsational period was also found to be increasing. This is the first detection of a historical turnaround of a pulsational amplitude change in the Cepheids. We clearly find the presence of additional RV variations on a timescale of about 119 days and an amplitude of about ±138 m s⁻¹, which is quasi-periodic rather than strictly periodic. From our data, we do not confirm the presence of the variation on a timescale of 34–45 days found in the RV data obtained in the 1980s and 1990s. We assume that both the 119 day quasi-periodic, noncoherent variations found in our data and the 34–45 day variations found previously can be caused by the 119 day rotation periods of Polaris and by surface inhomogeneities such as single- or multiple-spot configuration varying with time.

We determine an absolute calibration for the Multiband Imaging Photometer for Spitzer 24 μm band and recommend adjustments to the published calibrations for Two Micron All Sky Survey (2MASS), Infrared Array Camera (IRAC), and IRAS photometry to put them on the same scale. We show that consistent results are obtained by basing the calibration on either an average A0V star spectral energy distribution (SED), or by using the absolutely calibrated SED of the Sun in comparison with solar-type stellar photometry (the solar analog method). After the rejection of a small number of stars with anomalous SEDs (or bad measurements), upper limits of ∼1.5% root mean square (rms) are placed on the intrinsic infrared (IR) SED variations in both A-dwarf and solar-type stars. These types of stars are therefore suitable as general-purpose standard stars in the IR. We provide absolutely calibrated SEDs for a standard zero magnitude A star and for the Sun to allow extending this work to any other IR photometric system. They allow the recommended calibration to be applied from 1 to 25 μm with an accuracy of ∼2%, and with even higher accuracy at specific wavelengths such as 2.2, 10.6, and 24 μm, near which there are direct measurements. However, we confirm earlier indications that Vega does not behave as a typical A0V star between the visible and the IR, making it problematic as the defining star for photometric systems. The integration of measurements of the Sun with those of solar-type stars also provides an accurate estimate of the solar SED from 1 through 30 μm, which we show agrees with theoretical models.

We present the results of our ongoing radial-velocity (RV) survey of the old (7 Gyr) open cluster NGC 188. Our WIYN 3.5 m data set spans a time baseline of 11 years, a magnitude range of 12 ⩽ V ⩽ 16.5 (1.18–0.94 M_☉), and a 1° diameter region on the sky. With the addition of a Domain Astrophysical Observatory data set we extend our bright limit to V = 10.8 and, for some stars, extend our time baseline to 35 years. Our magnitude limits include solar-mass main-sequence stars, subgiants, giants, and blue stragglers (BSs), and our spatial coverage extends radially to 17 pc (∼13 core radii). For the WIYN data we present a detailed description of our data reduction process and a thorough analysis of our measurement precision of 0.4 km s⁻¹ for narrow-lined stars. We have measured radial velocities for 1046 stars in the direction of NGC 188, and have calculated RV membership probabilities for stars with ⩾3 measurements, finding 473 to be likely cluster members. We detect 124 velocity-variable cluster members, all of which are likely to be dynamically hard-binary stars. Using our single member stars, we find an average cluster radial velocity of −42.36 ± 0.04 km s⁻¹. We use our precise RV and proper-motion membership data to greatly reduce field-star contamination in our cleaned color–magnitude diagram, from which we identify six stars of note that lie far from a standard single-star isochrone. We present a detailed study of the spatial distribution of cluster-member populations, and find the binaries to be centrally concentrated, providing evidence for the presence of mass segregation in NGC 188. We observe the BSs to populate a bimodal spatial distribution that is not centrally concentrated, suggesting that we may be observing two populations of BSs in NGC 188, including a centrally concentrated distribution as well as a halo population. Finally, we find NGC 188 to have a global RV dispersion of 0.64 ± 0.04 km s⁻¹, which may be inflated by up to 0.23 km s⁻¹ from unresolved binaries. When corrected for unresolved binaries, the NGC 188 RV dispersion has a nearly isothermal radial distribution. We use this mean-corrected velocity dispersion to derive a virial mass of 2300 ± 460 M_☉ .

We report near- and mid-infrared (IR) images of the southern hemisphere dark cloud DC 296.2–3.6 associated with IRAS 11431 − 6516. The K_s and L' images show the presence of an IR nebulosity at the center of the dark cloud (DC). From the analysis of the near-IR color–color diagrams we have identified a young stellar population in the region. Five of these young stellar objects, here named A, B, C, D, and E, were also detected in the mid-IR. Sources B, D, and E are Class I–II T Tauri as suggested by the analysis of their spectral energy distributions. In addition, source E shows a long-term near-IR variability. The near-IR color–color diagrams indicate the presence of circumstellar dust envelope in sources A, B, D, and E, while the fit of SEDs of the intermediate- and low-mass objects A and B with a radiation transfer model including infalling envelope+disk+central source suggests circumstellar disks around these two objects. These results indicate that DC 296.2–3.6, located in the far Carina arm, is associated with an embedded cluster of low-mass young stellar objects.

We investigate the expected increase in the rotation rate of post-main-sequence stars as they expand and ingest orbiting planets. This phenomenon is expected to occur when the stellar radius becomes larger than the planet's periastron distance. We calculate the expected frequency of planet ingestion during the red giant, horizontal branch (HB), and early asymptotic giant branch phases for planets of mass $m_{\rm p} \ge 1 {{\cal M}_{\rm J}}$. We also calculate the probability of observing anomalous rotation rates in a population of solar metallicity giants as a function of stellar mass and evolutionary stage. Planet ingestion is most easily detectable in a solar mass HB star, with a probability of about 1% for solar-neighborhood metallicity. Our analysis is based on the observed distribution of mass, eccentricity, semimajor axis for extrasolar planets around solar-type main-sequence stars, on stellar evolution models, and on the typical observed rotation rates observed in a sample of solar-neighborhood giants.

A line ratio of 3d–2p transition lines in boron-like spectra of Si x, S xii, Ar xiv, and Fe xxii has been investigated. Collisional-radiative model calculations reveal that the line ratio is sensitive to the electron density in ranges of n_e = 4.0 × 10⁷–3.0 × 10¹⁰ cm⁻³, 4.0 × 10⁸–3.0 × 10¹¹ cm⁻³, 3.0 × 10⁹–4.0 × 10¹² cm⁻³, and 2.0 × 10¹²–3.0 × 10¹⁵ cm⁻³, respectively. This complements the K-shell diagnostics of helium-like ions. By a comparison between the predicted and the measured values, effective electron densities in the electron beam ion trap (EBIT) plasmas, performed by Lepson and collaborators at the Lawrence Livermore EBIT, are estimated to be n_e = 3.4^+0.8_−0.6 × 10¹⁰ cm⁻³ and 5.6^+1.0_−1.1 × 10¹⁰ cm⁻³ for sulfur and argon plasmas, respectively. In the case of argon, a good agreement is shown with the actual electron density derived from an N vi K-shell spectrum. We further explore the 3d–2p transition lines of Si x and S xii in the stellar coronal spectra measured with the low energy transmission grating spectrometer combined with a high resolution camera on board the Chandra X-Ray Observatory. The constrained electron densities show a good agreement with those determined from C v and O vii K-shell spectra.

Among various formulations to integrate rotational motions numerically, we recommend the integration of the combination of Euler parameters and the angular velocity components referred to the body-fixed reference frame with the renormalization of the Euler parameters at every integration step. This is because the formulation proposed is not only simple and regular, but also stable and precise in studying rotational motions numerically. Its efficiency is not degraded even for almost uniform rotations if their nominal uniform part is separated by Encke's method.

We have carried out near-IR/optical observations to examine star formation toward a bright-rimmed cometary globule (BRC37) facing the exciting star(s) of an H ii region (IC1396) containing an IRAS source, which is considered to be an intermediate-mass protostar. With slitless spectroscopy we detected ten Hα emission stars around the globule, six of which are near the tip of the globule and are aligned along the direction to the exciting stars. There is evidence that this alignment was originally toward an O9.5 star, but has evolved to align toward a younger O6 star when that formed. Near-IR and optical photometry suggests that four of these six stars are low-mass young stellar objects (YSOs) with masses of ∼0.4 M_☉. Their estimated ages of ∼1 Myr indicate that they were formed at the tip in advance of the formation of the IRAS source. Therefore, it is likely that sequential star formation has been taking place along the direction from the exciting stars toward the IRAS source, due to the UV impact of the exciting star(s). Interestingly, one faint, Hα emission star, which is the closest to the exciting star(s), seems to be a young brown dwarf that was formed by the UV impact in advance of the formation of other YSOs at the tip.

Often a newly discovered near-Earth asteroid is linked to old observations of a formerly lost object. This orbital identification is done using a standard dynamical model that accounts for gravitational perturbations from planets and relativistic effects. Here we report the first case where such an identification requires consideration of the Yarkovsky effect, a tiny non-gravitational perturbation due to the recoil of thermal radiation from the body. Moreover, this implies that the Yarkovsky force is revealed in the orbital motion of the body, asteroid 152563 (1992 BF), only the second case so far. Orbital fits indicate a drift in the orbital semi-major axis of −(10.7 ± 0.7) × 10⁻⁴ AU Myr⁻¹, which we ascribe to Yarkovsky forces. This yields a correlated constraint of physical parameters such as the obliquity, rotation rate, surface thermal inertial, and bulk density. The magnitude and direction of drift point to an obliquity in excess of 120°. Observations taken during 2011 and subsequent close encounters with the Earth might help establish rotation parameters and thereby constrain thermal inertia of 1992 BF, thus making the Yarkovsky strength a measure of this asteroid's bulk density.

We present an analysis of high-dispersion echelle spectra of four giant stars in the 4 Gyr open cluster NGC 7142 obtained with the KPNO 4 m telescope. Abundances were determined relative to the bright, mildly metal-poor giant Arcturus. NGC 7142 is found to have a mean [Fe/H] = +0.14 ± 0.01 (standard deviation) relative to the Sun, with an estimated uncertainty of 0.12 dex per star. Oxygen abundances, determined from the forbidden [O i] λ6300 Å feature, are slightly sub-solar, while the α-elements are slightly enhanced to ∼0.1 dex above solar. Na and Al are enhanced ∼0.2 dex relative to scaled solar abundances. These abundance patterns are similar to those reported for other open clusters in the literature, and suggest that the Galactic disk underwent rapid chemical enrichment in early phases of its evolution. Finally, comparison of these results to those found from lower-dispersion WIYN 3.5 m HYDRA spectra of the cluster stars shows them to be comparable within the errors, verifying that detailed abundance analyses of HYDRA spectra are reliable and can be directly compared to analyses of higher-dispersion data.

We present the density structure of dark matter in the outer parts (to about seven effective radii) of the elliptical galaxy NGC 4636, from the radial velocities of 174 globular clusters (GCs), using the non-parametric, inverse algorithm CHASSIS. We find the galaxy to be rich in dark matter, with R-band mass-to-light ratios (M/L) rising to about 30, at nearly 4R_e; the K-band M/L at about 3R_e is found to be nearly 10. The result does not depend on applying the method to the red and blue GCs separately. This estimate of M/L is higher than the previous analysis from the same kinematic data. We also find that the dark-matter distribution is highly concentrated toward the inner halo.

Reported here are high-resolution 6 cm measurements of the adolescent supernova remnant (SNR) Kesteven 69 made with the hybrid BnC configuration of the Very Large Array. Several three-field mosaics of the polarized and total intensity have been used to study this SNR. These investigations lead to a coherent picture of this region. The expanding shock defines an outer rim of high total intensity, suggesting the front is running into large dense clouds with random magnetic field directions. The SNR consists of predominantly of two types of regions, those with high total and relatively weak polarized emission and those with relatively weak total and strong polarized emission. This morphology can be generally explained by the number of clouds with organized magnetic field along the line of sight. Within this SNR there are regions where the field is varying from radial to tangential. As the SN shock encounters clouds, magnetic fields within clouds will strongly affect cloud dynamics.

The triple radio source detected in association with the luminous infrared source IRAS 16547-4247 has previously been studied with high angular resolution and high sensitivity with the Very Large Array at 3.6 cm wavelength. In this paper, we present new 3.6 cm observations taken 2.68 years after the first epoch that allow a search for variability and proper motions, as well as the detection of additional faint sources in the region. We do not detect proper motions along the axis of the outflow in the outer lobes of this source at a 4σ upper limit of ∼160 km s⁻¹. This suggests that these lobes are probably working surfaces where the jet is interacting with a denser medium. However, the brightest components of the lobes show evidence of precession, at a rate of 008 yr⁻¹ clockwise in the plane of the sky. It may be possible to understand the distribution of almost all the identified sources as the result of ejecta from a precessing jet. The core of the thermal jet shows significant variations in flux density and morphology. We compare this source with other jets in low- and high-mass young stars and suggest that the latter can be understood as a scaled-up version of the former.

Young stars orbiting in the gravitational potential well of forming star clusters pass through the cluster's dense molecular gas and can experience Bondi–Hoyle accretion from reservoirs outside their individual protostellar cloud cores. Accretion can occur for several million years after the stars form, but before the cluster disperses. This accretion is predominantly onto the disk and not the star. N-body simulations of stars orbiting in three young model clusters containing 30, 300, and 3000 stars are presented. The simulations include the gravitational potential of the molecular gas which smoothly disperses over time. The clusters have a star-formation efficiency of 33% and a radius of 0.22 pc. We find that the disks surrounding solar-mass stars in the N = 30 cluster accretes ∼0.01 M_☉ (1 minimum-mass solar nebula, MMSN) per Myr, with a 1σ width of 50 times due to variations in initial stellar positions and velocities within the cluster. The accretion rate scales as M^2.1±0.1 for stars of mass M. The accretion rate is ∼5 times lower for the N = 3000 cluster, due to its higher stellar velocities and higher temperature. The Bondi–Hoyle accretion rates onto the disks are several times lower than accretion rates observed directly onto young stars (e.g., Muzerolle et al. 2005): these two accretion rates follow the same M² behavior and may be related. The accreted disk mass is large enough that it may have a substantial and unappreciated effect on disk structure and the formation of planetary systems. We discuss a variety of implications of this process, including its effect on metallicity differences between cluster stars, compositional differences between a star and its disk, the formation of terrestrial and gas-giant planets, and isotopic anomalies observed in our solar system.

We discuss the implementation of a new regular algorithm for simulation of the gravitational few-body problem. The algorithm uses components from earlier methods, including the chain structure, the logarithmic Hamiltonian, and the time-transformed leapfrog. This algorithmic regularization code, AR-CHAIN, can be used for the normal N-body problem, as well as for problems with softened potentials and/or with velocity-dependent external perturbations, including post-Newtonian terms, which we include up to order PN2.5. Arbitrarily extreme mass ratios are allowed. Only linear coordinate transformations are used and thus the algorithm is somewhat simpler than many earlier regularized schemes. We present the results of performance tests which suggest that the new code is either comparable in performance or superior to the existing regularization schemes based on the Kustaanheimo–Stiefel (KS) transformation. This is true even for the two-body problem, independent of eccentricity. An important advantage of the new method is that, contrary to the older KS-CHAIN code, zero masses are allowed. We use our algorithm to integrate the orbits of the S stars around the Milky Way supermassive black hole for one million years, including PN2.5 terms and an intermediate-mass black hole. The three S stars with shortest periods are observed to escape from the system after a few hundred thousand years.

We present Spitzer infrared, Galaxy Evolution Explorer UV, and Sloan Digitized Sky Survey and Southeastern Association for Research in Astronomy optical images of the peculiar interacting galaxy pair Arp 285 (NGC 2856/4), and compare with a new numerical model of the interaction. We estimate the ages of clumps of star formation in these galaxies using population synthesis models, carefully considering the uncertainties on these ages. This system contains a striking example of "beads on a string": a series of star-formation complexes ∼1 kpc apart. These "beads" are found in a tail-like feature that is perpendicular to the disk of NGC 2856, which implies that it was formed from material accreted from the companion NGC 2854. The extreme blueness of the optical/UV colors and redness of the mid-infrared colors implies very young stellar ages (∼4–20 Myr) for these star-forming regions. Spectral decomposition of these "beads" shows excess emission above the modeled stellar continuum in the 3.6 μm and 4.5 μm bands, indicating either contributions from interstellar matter to these fluxes or a second older stellar population. These clumps have −12.0 < M_B< −10.6, thus they are less luminous than most dwarf galaxies. Our model suggests that bridge material falling into the potential of the companion overshoots the companion. The gas then piles up at apogalacticon before falling back onto the companion, and star formation occurs in the pile-up. There was a time delay of ∼500 Myr between the point of closest approach between the two galaxies and the initiation of star formation in this feature. A luminous (M_B ∼ −13.6) extended (FWHM ∼ 1.3 kpc) "bright spot" is visible at the northwestern edge of the NGC 2856 disk, with an intermediate stellar population (400–1500 Myr). Our model suggests that this feature is part of a expanding ripple-like "arc" created by an off-center ring-galaxy-like collision between the two disks.

In this paper we present measurements of velocity dispersions and Lick indices for 509 galaxies in the local universe, based on high signal-to-noise, long-slit spectra obtained with the 1.52 m ESO telescope at La Silla. The conversion of our measurements into the Lick/IDS system was carried out following the general prescription of Worthey & Ottaviani in 1997. Comparisons of our measurements with those of other authors show, in general, good agreement. We also examine the dependence between these indices (e.g., Hβ, Mg₂, Fe5270, and NaD) and the central velocity dispersion (σ₀), and we find that they are consistent with those previously reported in the literature. Benefiting from the relatively large size of the sample, we are able to investigate the dependence of these relations on morphology and environment, here represented by the local galaxy density. We find that for metallic lines these relations show no significant dependence on environment or morphology, except in the case of NaD, which shows distinct behavior for E and S0. On the other hand, the Hβ–log σ₀ shows a significant difference as a function of the local density of galaxies, which we interpret as being caused by the truncation of star formation in high-density environments. Comparing our results with those obtained by other authors, we find a few discrepancies, adding to the ongoing debate about the nature of these relations. Finally, we report that the scatter of the Mg indices versus σ₀ relations correlates with Hβ, suggesting that age may contribute to the scatter. Furthermore, this scatter shows no significant dependence on morphology or environment. Our results are consistent with the currently popular downsizing model, where low-mass galaxies have an extended star-formation history, except for those located in high-density regions.

The Little Red Spot (LRS) in Jupiter's atmosphere was investigated in unprecedented detail by the New Horizons spacecraft together with the Hubble Space Telescope (HST) and the Very Large Telescope (VLT). The LRS and the larger Great Red Spot (GRS) of Jupiter are the largest known atmospheric storms in the solar system. Originally a white oval, the LRS formed from the mergers of three smaller storms in 1998 and 2000, and became as red as the GRS between 2005 and 2006. Here we show that circulation and wind speeds in the LRS have increased substantially since the Voyager and Galileo eras when the oval was white. The maximum tangential velocity of the LRS is now 172 ± 18 m s⁻¹, close to the highest values ever seen in the GRS, which has also evolved both in size and maximum wind speed. The cloud-top altitudes of the GRS and LRS are similar, both storms extending much higher in the atmosphere than other Jovian anti-cyclonic systems. The similarities in wind speeds, cloud morphology, and coloring suggest a common dynamical mechanism explaining the reddening of the two largest anticyclonic systems on Jupiter. These storms will not be observed again from close range until at least 2016.

We present a large sample of 501 radio-selected BL Lac candidates from a combination of Sloan Digital Sky Survey (SDSS) Data Release 5 optical spectroscopy and the Faint Images of the Radio Sky at Twenty cm (FIRST) radio survey; this is one of the largest BL Lac samples yet assembled, and each object emerges with homogeneous data coverage. Each candidate is detected in the radio from FIRST and confirmed in SDSS optical spectroscopy to have (1) no emission feature with measured rest-equivalent width larger than 5 Å and (2) no measured Ca ii H/K depression larger than 40%. We subdivide our sample into 426 higher-confidence candidates and 75 lower-confidence candidates. We argue that contamination from other classes of objects that formally pass our selection criteria is small, and we identify a few very-rare radio active galactic nuclei (AGNs) with unusual spectra that are probably related to broad absorption line quasars. About one-fifth of our sample were known BL Lac objects prior to the SDSS. A preliminary analysis of the sample generally supports the standard beaming paradigm. While we recover sizable numbers of low-energy and intermediate-energy cutoff BL Lac objects (LBLs and IBLs, respectively), there are indications of a potential bias toward recovering high-energy cutoff BL Lac objects (HBLs) from SDSS spectroscopy. Such a large sample may eventually provide new constraints on BL Lac unification models and their potentially peculiar cosmic evolution; in particular, our sample contains a significant number of higher redshift objects, a sub-population for which the standard paradigm has yet to be rigorously constrained.

The Australia Telescope Hubble Deep Field–South (ATHDF-S) survey of the Hubble Deep Field–South (HDF-S) reaches sensitivities of ∼10 μJy at 1.4, 2.5, 5.2, and 8.7 GHz, making the ATHDF-S one of the deepest surveys ever performed with the Australia Telescope Compact Array (ATCA). Here, we present the optical identifications of the ATHDF-S radio sources using data from the literature. We find that ∼66% of the radio sources have optical counterparts to I = 23.5 mag. Deep Hubble Space Telescope (HST) imaging of the area identifies a further 12% of radio sources. We present new spectroscopic observations for 98 of the radio sources and supplement these spectroscopic redshifts with photometric ones calculated from five-band optical imaging. The host galaxy colors and radio-to-optical ratios indicate that low-luminosity (or "radio-quiet") active galactic nuclei make up a significant proportion of the sub-mJy radio population, a result which is in accordance with a number of other deep radio studies. The radio-to-optical ratios of the bright (S_1.4 GHz > 1 mJy) sources are consistent with a bimodal distribution.

We present the observational results of a near-infrared survey of a large sample of Class I protostars designed to determine the Class I binary separation distribution from ∼100 AU to ∼5000 AU. We have selected targets from a new sample of 267 nearby candidate Class I objects. This sample is well understood, consists of mostly Class I young stellar objects (YSOs) within 1 kpc, has targets selected from the whole sky, and is not biased by previous studies of star formation. We have observed 189 Class I YSOs north of δ = −40° at the H, K, and L' bands, with a median angular resolution of 033 at L'. We determine our detection limit for close binary companions by observing artificial binaries. We choose a contrast limit and an outer detection limit to minimize contamination and to ensure that a candidate companion is gravitationally bound. Our survey uses observations at the L' rather than the K band for the detection of binary companions since there is less scattered light and better seeing at L'. This paper presents the positions of our targets, the near-IR photometry of sources detected in our fields at L', as well as the observed properties of the 89 detected companions (73 of which are newly discovered). Although we have chosen contrast and separation limits to minimize contamination, we expect that there are about six stars identified as binary companions that are due to contamination. Finder charts at L' for each field are shown to facilitate future studies of these objects.

We present the Class I protostellar binary separation distribution based on the data tabulated in a companion paper. We verify the excess of Class I binary stars over solar-type main-sequence stars in the separation range from 500 AU to 4500 AU. Although our sources are in nearby star-forming regions distributed across the entire sky (including Orion), none of our objects are in a high stellar density environment. A log–normal function, used by previous authors to fit the main-sequence and T Tauri binary separation distributions, poorly fits our data, and we determine that a log–uniform function is a better fit. Our observations show that the binary separation distribution changes significantly during the Class I phase, and that the binary frequency at separations greater than 1000 AU declines steadily with respect to spectral index. Despite these changes, the binary frequency remains constant until the end of the Class I phase, when it drops sharply. We propose a scenario to account for the changes in the Class I binary separation distribution. This scenario postulates that a large number of companions with a separation greater than ∼1000 AU were ejected during the Class 0 phase, but remain gravitationally bound due to the significant mass of the Class I envelope. As the envelope dissipates, these companions become unbound and the binary frequency at wide separations declines. Circumstellar and circumbinary disks are expected to play an important role in the orbital evolution at closer separations. This scenario predicts that a large number of Class 0 objects should be non-hierarchical multiple systems, and that many Class I young stellar objects (YSOs) with a widely separated companion should also have a very close companion. We also find that Class I protostars are not dynamically pristine, but have experienced dynamical evolution before they are visible as Class I objects. Our analysis shows that the Class I binary frequency and the binary separation distribution strongly depend on the star-forming environment.