Ancient eruptions of Eta Carinae: A tale written in proper motions

We analyze eight epochs of Hubble Space Telescope H$\alpha$+[N II] imaging of Eta Carinae's outer ejecta. Proper motions of nearly 800 knots reveal that the detected ejecta are divided into three apparent age groups, dating to around 1250 A.D., to around 1550 A.D., and to during or shortly before the Great Eruption of the 1840s. Ejecta from these groups reside in different locations and provide a firm constraint that Eta Car experienced multiple major eruptions prior to the 19th century. The 1250 and 1550 events did not share the same axisymmetry as the Homunculus; the 1250 event was particularly asymmetric, even one-sided. In addition, the ejecta in the S ridge, which have been associated with the Great Eruption, appear to predate the ejection of the Homunculus by several decades. We detect essentially ballistic expansion across multiple epochs. We find no evidence for large-scale deceleration of the observed knots that could power the soft X-ray shell by plowing into surrounding material, suggesting that the observed X-rays arise instead from fast, rarefied ejecta from the 1840s overtaking the older dense knots. Early deceleration and subsequent coasting cannot explain the origin of the older outer ejecta---significant episodic mass loss prior to the 19th century is required. The timescale and geometry of the past eruptions provide important constraints for any theoretical physical mechanisms driving Eta Car's behavior. Non-repeating mechanisms such as the merger of a close binary in a triple system would require additional complexities to explain the observations.Comment: 14 pages, 11 figures, accepted for publication in MNRA

Proper motions of collimated jets from intermediate-mass protostars in the Carina Nebula

We present proper motion measurements of 37 jets and HH objects in the Carina Nebula measured in two epochs of H$\alpha$ images obtained $\sim 10$ yrs apart with HST/ACS. Transverse velocities in all but one jet are faster than $\gtrsim 25$ km s$^{-1}$, confirming that the jet-like H$\alpha$ features identified by Smith et al. (2010) trace outflowing gas. Proper motions constrain the location of the jet-driving source and provide kinematic confirmation of the intermediate-mass protostars that we identify for 20/37 jets. Jet velocities do not correlate with the estimated protostar mass and embedded driving sources do not have slower jets. Instead, transverse velocities (median $\sim 75$ km s$^{-1}$) are similar to those in jets from low-mass stars. Assuming a constant velocity since launch, we compute jet dynamical ages (median $\sim 10^4$ yr). If continuous emission from inner jets traces the duration of the most recent accretion bursts, then these episodes are sustained longer (median $\sim 700$ yr) than the typical decay time of an FU Orionis outburst. These jets can carry appreciable momentum that may be injected into the surrounding environment. The resulting outflow force, $dP/dt$, lies between that measured in low- and high-mass sources, despite the very different observational tracers used. Smooth scaling of the outflow force argues for a common physical process underlying outflows from protostars of all masses. This latest kinematic result adds to a growing body of evidence that intermediate-mass star formation proceeds like a scaled-up version of the formation of low-mass stars.Comment: accepted to MNRAS, 29 pages, 30 figures, 3 table

Proper motions of five OB stars with candidate dusty bow shocks in the Carina Nebula

We constrain the proper motions of five OB stars associated with candidate stellar wind bow shocks in the Carina Nebula using HST ACS imaging over 9--10 year baselines. These proper motions allow us to directly compare each star's motion to the orientation of its candidate bow shock. Although these stars are saturated in our imaging, we assess their motion by the shifts required to minimize residuals in their Airy rings. The results limit the direction of each star's motion to sectors less than 90 degrees wide. None of the five stars are moving away from the Carina Nebula's central clusters as runaway stars would be, confirming that a candidate bow shock is not necessarily indicative of a runaway star. Two of the five stars are moving tangentially relative to the orientation of their candidate bow shocks, both of which point at the OB cluster Trumpler 14. In these cases, the large-scale flow of the interstellar medium, powered by feedback from the cluster, appears to dominate over the motion of the star in producing the observed candidate bow shock. The remaining three stars all have some component of motion toward the central clusters, meaning that we cannot distinguish whether their candidate bow shocks are indicators of stellar motion, of the flow of ambient gas, or of density gradients in their surroundings. In addition, these stars' lack of outward motion hints that the distributed massive-star population in Carina's South Pillars region formed in place, rather than migrating out from the association's central clusters.Comment: 13 pages, 5 figures, accepted for publication in MNRA

Additional Massive Binaries in the Cygnus OB2 Association

We report the discovery and orbital solutions for two new OB binaries in the Cygnus OB2 Association, MT311 (B2V+B3V) and MT605 (B0.5V+B2.5:V). We also identify the system MT429 as a probable triple system consisting of a tight eclipsing 2.97 day B3V+B6V pair and a B0V at a projected separation of 138 AU. We further provide the first spectroscopic orbital solutions to the eclipsing, double-lined, O-star binary MT696 (O9.5V+B1:V), the double-lined, early B binary MT720 (B0-1V+B1-2V), and the double-lined, O-star binary MT771 (O7V+O9V). These systems exhibit orbital periods between 1.5 days and 12.3 days, with the majority having P<6 days. The two new binary discoveries and six spectroscopic solutions bring the total number of known massive binaries in the central region of the Cygnus OB2 Association to 20, with all but two having full orbital solutions.Comment: 42 pages, 16 figures, accepted for publication in the Astrophysical Journal, minor typos have been fixe

A radial velocity survey of the Carina Nebula's O-type stars

We have obtained multi-epoch observations of 31 O-type stars in the Carina Nebula using the CHIRON spectrograph on the CTIO/SMARTS 1.5-m telescope. We measure their radial velocities to 1-2 km s(-1) precision and present new or updated orbital solutions for the binary systems HD 92607, HD 93576, HDE 303312, and HDE 305536. We also compile radial velocities from the literature for 32 additional O-type and evolved massive stars in the region. The combined data set shows a mean heliocentric radial velocity of 0.6 km s(-1). We calculate a velocity dispersion of <= 9.1 km s(-1), consistent with an unbound, substructured OB association. The Tr 14 cluster shows a marginally significant 5 km s(-1) radial velocity offset from its neighbour Tr 16, but there are otherwise no correlations between stellar position and velocity. The O-type stars in Cr 228 and the South Pillars region have a lower velocity dispersion than the region as a whole, supporting a model of distributed massive star formation rather than migration from the central clusters. We compare our stellar velocities to the Carina Nebula's molecular gas and find that Tr 14 shows a close kinematic association with the Northern Cloud. In contrast, Tr 16 has accelerated the Southern Cloud by 10-15 km s(-1), possibly triggering further massive star formation. The expansion of the surrounding HII region is not symmetric about the O-type stars in radial velocity space, indicating that the ionized gas is constrained by denser material on the far side.Commonwealth of AustraliaThis item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]