HST astrometry in the Orion Nebula Cluster: census of low-mass runaways

We present a catalog of high-precision proper motions in the Orion Nebula Cluster (ONC), based on Treasury Program observations with the Hubble Space Telescope's (HST) ACS/WFC camera. Our catalog contains 2,454 objects in the magnitude range of $14.2<m_{\rm F775W}<24.7$, thus probing the stellar masses of the ONC from $\sim$0.4 $M_\odot$ down to $\sim$0.02 $M_\odot$ over an area of $\sim$550 arcmin$^2$. We provide a number of internal velocity dispersion estimates for the ONC that indicate a weak dependence on the stellar location and mass. There is good agreement with the published velocity dispersion estimates, although nearly all of them (including ours at $\sigma_{v,x}=0.94$ and $\sigma_{v,y}=1.25$ mas yr$^{-1}$) might be biased by the overlapping young stellar populations of Orion A. We identified 4 new ONC candidate runaways based on HST and the Gaia DR2 data, all with masses less than $\sim$1 $M_\odot$. The total census of known candidate runaway sources is 10 -- one of the largest samples ever found in any Milky Way open star cluster. Surprisingly, none of them has the tangential velocity exceeding 20 km s$^{-1}$. If most of them indeed originated in the ONC, it may compel re-examination of dynamical processes in very young star clusters. It appears that the mass function of the ONC is not significantly affected by the lost runaways.Comment: 16 pages, 10 figures, 5 tables. Accepted for publication in A

A Search for a Surviving White Dwarf Companion in SN 1006

Multiple channels have been proposed to produce Type Ia supernovae, with many scenarios suggesting that the exploding white dwarf accretes from a binary companion pre-explosion. In almost all cases, theory suggests that this companion will survive. However, no such companion has been unambiguously identified in ancient supernova remnants -- possibly falsifying the accretion scenario. Existing surveys, however, have only looked for stars as faint as $\approx 0.1 L_\odot$ and thus would have missed a surviving white dwarf companion. In this work, we present very deep DECAM imaging $(u, g, r, z)$ of the Type Ia supernova remnant SN 1006 specifically to search for a potential surviving white dwarf companion. We find no object within the inner third of the SN 1006 remnant that is consistent with a relatively young cooling white dwarf. We find that if there is a companion white dwarf, it must have formed long ago and cooled undisturbed for $> 10^8$ yr to be consistent with the redder objects in our sample. We conclude that our findings are consistent with the complete destruction of the secondary (such as in a merger) or an unexpectedly cool and thus very dim surviving companion white dwarf.Comment: 9 pages, 5 figure, submitted to MNRAS - comments welcom

Private Meetings Between Firm Managers and Outside Investors: The European Paradigm

Institutional ownership of listed companies has grown significantly, leading to an increase in ownership concentration in the European Union. Under the current context of re-concentrated ownership, institutional shareholders are expected, also in Europe, to play a more active role in corporate governance and to exert influence on the company’s strategies. Within such a corporate governance landscape institutional investor engagement is becoming a distinctive feature of corporate governance of European listed companies. In particular, board-shareholder dialogue is a key engagement tool and is essential in order to enable institutional investors to fulfil their stewardship functions. Board-shareholder dialogue is also core to listed companies’ communication strategies, since the growing demand for engagement by institutional investors has rendered traditional investor relations insufficient. Nevertheless, private meetings between directors and institutional investors raise concerns with respect to the financial markets law framework in the EU. In particular, the EU market abuse regime and the related principle of equal treatment for shareholders seem to hinder dialogue between directors and key shareholders. Against this background this Article shows that legal constraints deriving from EU financial markets law do not hamper institutional investor engagement. Furthermore, based on recommendations from corporate governance and stewardship codes as well as good practice standards drafted by corporate governance experts and institutions, it outlines an innovative practical framework that reduces the risk of violating disclosure rules and fosters board-shareholder engagement. In doing so, the Article provides theoretical and practical insights that can help to make institutional investor engagement more effective also in non-European countries

HST survey of the Orion Nebula Cluster in the H2_2O 1.4 μ\mum absorption band: I. A census of substellar and planetary mass objects

In order to obtain a complete census of the stellar and sub-stellar population, down to a few M$_{Jup}$ in the $\sim1$ Myr old Orion Nebula Cluster, we used the infrared channel of the Wide Field Camera 3 of the Hubble Space Telescope with the F139M and F130N filters. These bandpasses correspond to the $1.4 \mu$m H$_2$O absorption feature and an adjacent line-free continuum region. Out of $4,504$ detected sources, $3,352$ (about $75\%$) appear fainter than m$_{130}=14$ (Vega mag) in the F130N filter, a brightness corresponding to the hydrogen-burning limit mass (M$\simeq 0.072 M_\odot$) at $\sim 1$ Myr. Of these, however, only $742$ sources have a negative F130M-139N color index, indicative of the presence of H$_2$O vapor in absorption, and can therefore be classified as bona-fide M and L dwarfs, with effective temperatures T$\lesssim 2850$ K at an assumed $1$ Myr cluster age. On our color-magnitude diagram, this population of sources with H$_2$O absorption appears clearly distinct from the larger background population of highly reddened stars and galaxies with positive F130M-F139N color index, and can be traced down to the sensitivity limit of our survey, m$_{130}\simeq 21.5$, corresponding to a $1$ Myr old $\simeq 3$M$_{Jup}$, planetary mass object under about 2 magnitudes of visual extinction. Theoretical models of the BT-Settl family predicting substellar isochrones of $1, 2$ and $3$ Myr (down to $\sim 1$M$_{Jup}$) fail to reproduce the observed H$_2$O color index at M$\lesssim 20$M$_{Jup}$. We perform a Bayesian analysis to determine extinction, mass and effective temperature of each sub-stellar member of our sample, together with its membership probability.Comment: Accepted for publication in the Astrophysical Journal. The resolution of several figures has been downgraded to comply with the size limit of arXiv submission

Private Meetings Between Firm Managers and Outside Investors: The European Paradigm

Institutional ownership of listed companies has grown significantly, leading to an increase in ownership concentration in the European Union. Under the current context of re-concentrated ownership, institutional shareholders are expected, also in Europe, to play a more active role in corporate governance and to exert influence on the company’s strategies. Within such a corporate governance landscape institutional investor engagement is becoming a distinctive feature of corporate governance of European listed companies. In particular, board-shareholder dialogue is a key engagement tool and is essential in order to enable institutional investors to fulfil their stewardship functions. Board-shareholder dialogue is also core to listed companies’ communication strategies, since the growing demand for engagement by institutional investors has rendered traditional investor relations insufficient. Nevertheless, private meetings between directors and institutional investors raise concerns with respect to the financial markets law framework in the EU. In particular, the EU market abuse regime and the related principle of equal treatment for shareholders seem to hinder dialogue between directors and key shareholders. Against this background this Article shows that legal constraints deriving from EU financial markets law do not hamper institutional investor engagement. Furthermore, based on recommendations from corporate governance and stewardship codes as well as good practice standards drafted by corporate governance experts and institutions, it outlines an innovative practical framework that reduces the risk of violating disclosure rules and fosters board-shareholder engagement. In doing so, the Article provides theoretical and practical insights that can help to make institutional investor engagement more effective also in non-European countries

Sub-percent Photometry: Faint DA White Dwarf Spectophotometric Standards for Astrophysical Observatories

We have established a network of 19 faint (16.5 mag $< V <$19 mag) northern and equatorial DA white dwarfs as spectrophotometric standards for present and future wide-field observatories. Our analysis infers SED models for the stars that are tied to the three CALSPEC primary standards. Our SED models are consistent with panchromatic Hubble Space Telescope ($HST$) photometry to better than 1%. The excellent agreement between observations and models validates the use of non-local-thermodynamic-equilibrium (NLTE) DA white dwarf atmospheres extinguished by interstellar dust as accurate spectrophotometric references. Our standards are accessible from both hemispheres and suitable for ground and space-based observatories covering the ultraviolet to the near infrared. The high-precision of these faint sources make our network of standards ideally suited for any experiment that has very stringent requirements on flux calibration, such as studies of dark energy using the Large Synoptic Survey Telescope (LSST) and the Wide-Field Infrared Survey Telescope ($WFIRST$).Comment: 46 pages, 23 figures, 8 tables, accepted for publication in ApJ

Are Passive Index Funds Active Owners? Corporate Governance Consequences of Passive Investing

The exponential rise of mutual funds designed to track stock indices has been one of the drivers behind the re-concentration of ownership of listed companies in the United States. Because of the high concentration of the passive index funds industry, the three leading passive fund managers—BlackRock, Vanguard, and State Street—make up an increasingly important component of the shareholder base of listed companies. In spite of this however, it remains questionable whether they are actually interested in playing an active role in the corporate governance of investee companies. In fact, although passive investors are, by definition, focused on the long term and, as such, should naturally be incentivized to monitor investee companies to improve their performance, passive fund managers generally adhere to a low-cost approach to voting and engagement to keep their fees low. Against this background, this Article provides an in-depth analysis of available evidence concerning the corporate governance role of passive investing and, taking the current EU institutional investor-driven corporate governance strategy as a reference, demonstrates the shortcomings of the regulatory approaches to institutional shareholder engagement focused mainly on short-termism. This Article therefore argues that, to promote more effective passive investor engagement, lawmakers, regulators, and corporate governance professionals should tackle cost-related issues more effectively. Moreover, pursuing this line of thought, it outlines an analytical framework of potential regulatory strategies aimed at reducing engagement-related costs to encourage passive index fund managers and, more generally, nonactivist institutional investors to play a more effective oversight role over investee companies