arXiv subjects

K. Z. Stanek

Publications and source records attributed to K. Z. Stanek.

At least 163 records · Page 9Linked to original sources

arXiv2013

The Impact of Metallicity on the Rate of Type Ia Supernovae

The metallicity of a star strongly affects both its evolution and the properties of the stellar remnant that results from its demise. It is generally accepted that stars with initial masses below ~8 M_sun leave behind white dwarfs and that some sub-population of these lead to Type Ia supernovae. However, it is often tacitly assumed that metallicity has no effect on the rate of SNe Ia. We propose that a consequence of the effects of metallicity is to significantly increase the SN Ia rate in lower-metallicity galaxies, in contrast to previous expectations. This is because lower-metallicity stars leave behind higher-mass white dwarfs, which should be easier to bring to explosion. We first model SN Ia rates in relation to galaxy masses and ages alone, finding that the elevation in the rate of SNe Ia in lower-mass galaxies measured by LOSS is readily explained. However, we then see that models incorporating this effect of metallicity agree just as well. Using the same parameters to estimate the cosmic SN Ia rate, we again find good agreement with data up to z~2. We suggest that this degeneracy warrants more detailed examination of host galaxy metallicities. We discuss additional implications, including for hosts of high-z SNe Ia, the SN Ia delay time distribution, super-Chandrasekhar SNe, and cosmology.

astro-ph.CO↗

arXiv2013

Probing the low-redshift star formation rate as a function of metallicity through the local environments of type II supernovae

Type II SNe can trace star formation to probe its global metallicity distribution at low-redshift. We present oxygen and iron abundance distributions of SN II progenitor regions that avoid many previous sources of bias. Because Fe (rather than O) abundance drives the late stage evolution of the massive stars that are the progenitors of CCSNe, and because Fe enrichment lags O enrichment, we find a general conversion from O abundance to Fe abundance. The distributions we present here are the best yet standard of comparison for evaluating how rare classes of SNe depend on progenitor metallicity. We measure the gas-phase O abundance of a representative subsample of the hosts of SNe II from the first-year PTF SN search, using a combination of SDSS spectra near the SN location (9) and new long slit spectroscopy (25). The median metallicity of these 34 hosts is 12+log(O/H) = 8.65, with a median error of 0.09. The median host galaxy stellar mass from fits to SDSS photometry is 10^9.9 solar masses. They do not show a systematic offset in metallicity or mass from a redshift-matched sample of the MPA/JHU value-added catalog. In contrast to previous SN host metallicity studies, this sample is drawn from a single, areal survey. SNe in the lowest-mass galaxies are not systematically excluded. The metallicity distribution we find is statistically indistinguishable from the metallicity distribution of SN II hosts found by targeted surveys and by samples from multiple surveys with different selection functions. Using the relationship between Fe and O abundances found for Milky Way disk, bulge, and halo stars, we translate our O abundance distribution of SN II environments into Fe abundance estimates. We find that though this sample spans only 0.65 dex in O abundance, the gap between the Fe and O abundance is 50% wider at the low-metallicity end of our sample than at the high-metallicity end. (abridged)

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arXiv2013

No Stripped Hydrogen in the Nebular Spectra of Nearby Type Ia Supernova 2011fe

A generic prediction of the single-degenerate model for Type Ia supernovae (SNe Ia) is that a significant amount of material will be stripped from the donor star (~0.5 Msun for a giant donor and ~0.15 Msun for a main sequence donor) by the supernova ejecta. This material, excited by gamma-rays from radioactive decay, would then produce relatively narrow (1000 km s-1) emission features observable once the supernova enters the nebular phase. Such emission has never been detected, which already provides strong constraints on Type Ia progenitor models. In this Letter we report the deepest limit yet on the presence of H-alpha emission originating from the stripped hydrogen in the nebular spectrum of a Type Ia supernova obtained using a high signal-to-noise spectrum of the nearby normal SN Ia 2011fe 274 days after B-band maximum light with the Large Binocular Telescope's Multi-Object Double Spectrograph. We put a conservative upper limit on the H-alpha flux of 3.14 x 10^-17 erg s-1 cm-2, which corresponds to a luminosity of 1.57 x 10^35 erg s-1. By scaling models from the literature, our flux limit translates into an upper limit of 0.001 Msun of stripped material. This is an order of magnitude stronger than previous limits. SN 2011fe was a typical Type Ia supernova, special only in its proximity, and we argue that lack of hydrogen emission in its nebular spectrum adds yet another strong constraint on the single degenerate class of models for SNe Ia.

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arXiv2013

Type Ia Single Degenerate Survivors Must Be Overluminous

In the single-degenerate (SD) channel of a Type Ia supernovae (SN Ia) explosion, a main-sequence (MS) donor star survives the explosion but it is stripped of mass and shock heated. An essentially unavoidable consequence of mass loss during the explosion is that the companion must have an overextended envelope after the explosion. While this has been noted previously, it has not been strongly emphasized as an inevitable consequence. We calculate the future evolution of the companion by injecting 2-6 10^47 ergs into the stellar evolution model of a 1 Msun donor star based on the post-explosion progenitors seen in simulations. We find that, due to the Kelvin-Helmholtz collapse of the envelope, the companion must become significantly more luminous (10 - 10^3 Lsun) for a long period of time (10^3 - 10^4 years). The lack of such a luminous "leftover" star in the LMC supernova remnant SNR 0609-67.5 provides another piece of evidence against the SD scenario. We also show that none of the stars proposed as the survivors of the Tycho supernova, including Tycho G, could plausibly be the donor star. Additionally, luminous donors closer than ~10 Mpc should be observable with the Hubble Space Telescope starting ~2 years post-peak. Such systems include SN 1937C, SN 1972E, SN 1986G, and SN 2011fe. Thus, the SD channel is already ruled out for at least two nearby SNe Ia and can be easily tested for a number of additional ones. We also discuss similar implications for the companions of core-collapse SNe.

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arXiv2012

Finding η Car Analogs in Nearby Galaxies Using Spitzer: I. Candidate Selection

The late-stage evolution of the most massive stars such as η Carinae is controlled by the effects of mass loss, which may be dominated by poorly understood eruptive mass ejections. Understanding this population is challenging because no true analogs of η Car have been clearly identified in the Milky Way or other galaxies. We utilize Spitzer IRAC images of 7 nearby (=<4 Mpc) galaxies to search for such analogs. We find 34 candidates with a flat or rising mid-IR spectral energy distributions towards longer mid-infrared wavelengths that emit 10^5 L_sun in the IRAC bands (3.6 to 8.0 micron) and are not known to be background sources. Based on our estimates for the expected number of background sources, we expect that follow-up observations will show that most of these candidates are not dust enshrouded massive stars, with an expectation of only 6+-6 surviving candidates. Since we would detect true analogs of η Car for roughly 200 years post-eruption, this implies that the rate of eruptions like η Car is less than the ccSN rate. It is possible, however, that every M > 40 M_sun star undergoes such eruptions given our initial results. In Paper II we will characterize the candidates through further analysis and follow-up observations, and there is no barrier to increasing the galaxy sample by an order of magnitude.

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arXiv2012

KELT-2Ab: A Hot Jupiter Transiting the Bright (V=8.77) Primary Star of a Binary System

We report the discovery of KELT-2Ab, a hot Jupiter transiting the bright (V=8.77) primary star of the HD 42176 binary system. The host is a slightly evolved late F-star likely in the very short-lived "blue-hook" stage of evolution, with $\teff=6148\pm48{\rm K}$, $\log{g}=4.030_{-0.026}^{+0.015}$ and $\feh=0.034\pm0.78$. The inferred stellar mass is $M_*=1.314_{-0.060}^{+0.063}$\msun\ and the star has a relatively large radius of $R_*=1.836_{-0.046}^{+0.066}$\rsun. The planet is a typical hot Jupiter with period $4.11379\pm0.00001$ days and a mass of $M_P=1.524\pm0.088$\mj\ and radius of $R_P=1.290_{-0.050}^{+0.064}$\rj. This is mildly inflated as compared to models of irradiated giant planets at the $\sim$4 Gyr age of the system. KELT-2A is the third brightest star with a transiting planet identified by ground-based transit surveys, and the ninth brightest star overall with a transiting planet. KELT-2Ab's mass and radius are unique among the subset of planets with $V<9$ host stars, and therefore increases the diversity of bright benchmark systems. We also measure the relative motion of KELT-2A and -2B over a baseline of 38 years, robustly demonstrating for the first time that the stars are bound. This allows us to infer that KELT-2B is an early K-dwarf. We hypothesize that through the eccentric Kozai mechanism KELT-2B may have emplaced KELT-2Ab in its current orbit. This scenario is potentially testable with Rossiter-McLaughlin measurements, which should have an amplitude of $\sim$44 m s$^{-1}$.

astro-ph.EP↗

arXiv2012

A Study of Cepheids in M81 with the Large Binocular Telescope (Efficiently Calibrated with HST)

We identify and phase a sample of 107 Cepheids with 10<P/days<100 in M81 using the LBT and calibrate their BVI mean magnitudes with archival HST data. The use of a ground-based telescope to identify and phase the Cepheids and HST only for the final calibration reduces the demand on HST by nearly an order of magnitude and yields Period-Luminosity (PL) relations with dispersions comparable to the best LMC samples. We fit the sample using the OGLE-II LMC PL relations and are unable to find a self-consistent distance for different band combinations or radial locations within M81. We can do so after adding a radial dependence to the PL zero point that corresponds to a luminosity dependence on metallicity of g_mu=-0.56+/-0.36 mag/dex. We find marginal evidence for a shift in color as a function of metallicity, distinguishable from the effects of extinction, of g_2=+0.07+/-0.03 mag/dex. We find a distance modulus for M81, relative to the LMC, of mu(M81-LMC)=9.39+/-0.14 mag, including uncertainties due to the metallicity corrections. This corresponds to a distance to M81 of 3.6+/-0.2 Mpc, assuming a LMC distance modulus of 18.41 mag. We carry out a joint analysis of M81 and NGC4258 Cepheids and simultaneously solve for the distance of M81 relative to NGC4258 and the metallicity corrections. Given the current data, the uncertainties of such joint fits are dominated by the relative metallicities and the abundance gradients rather than by measurement errors of the Cepheid magnitudes or colors. We find mu(M81-LMC)=9.40 (-0.11/+0.15) mag, mu(N4258-LMC)=11.08 (-0.17/+0.21) mag and mu(N4258-M81)=1.68+/-0.08 mag and joint metallicity corrections of g_mu=-0.62 (-0.35/+0.31) mag/dex and g_2=0.01+/-0.01 mag/dex. Quantitative analyses of Cepheid distances must take into account both the metallicity dependencies of the Cepheids and the uncertainties in the abundance estimates. (ABRIDGED)

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arXiv2012

Reverberation Mapping Results for Five Seyfert 1 Galaxies

We present the results from a detailed analysis of photometric and spectrophotometric data on five Seyfert 1 galaxies observed as a part of a recent reverberation mapping program. The data were collected at several observatories over a 140-day span beginning in 2010 August and ending in 2011 January. We obtained high sampling-rate light curves for Mrk 335, Mrk 1501, 3C120, Mrk 6, and PG2130+099, from which we have measured the time lag between variations in the 5100 Angstrom continuum and the H-beta broad emission line. We then used these measurements to calculate the mass of the supermassive black hole at the center of each of these galaxies. Our new measurements substantially improve previous measurements of MBH and the size of the broad line-emitting region for four sources and add a measurement for one new object. Our new measurements are consistent with photoionization physics regulating the location of the broad line region in active galactic nuclei.

astro-ph.CO↗

arXiv2012

KELT-1b: A Strongly Irradiated, Highly Inflated, Short Period, 27 Jupiter-mass Companion Transiting a mid-F Star

We present the discovery of KELT-1b, the first transiting low-mass companion from the wide-field Kilodegree Extremely Little Telescope-North (KELT-North) survey. The V=10.7 primary is a mildly evolved, solar-metallicity, mid-F star. The companion is a low-mass brown dwarf or super-massive planet with mass of 27.23+/-0.50 MJ and radius of 1.110+0.037-0.024 RJ, on a very short period (P=1.21750007) circular orbit. KELT-1b receives a large amount of stellar insolation, with an equilibrium temperature assuming zero albedo and perfect redistribution of 2422 K. Upper limits on the secondary eclipse depth indicate that either the companion must have a non-zero albedo, or it must experience some energy redistribution. Comparison with standard evolutionary models for brown dwarfs suggests that the radius of KELT-1b is significantly inflated. Adaptive optics imaging reveals a candidate stellar companion to KELT-1, which is consistent with an M dwarf if bound. The projected spin-orbit alignment angle is consistent with zero stellar obliquity, and the vsini of the primary is consistent with tidal synchronization. Given the extreme parameters of the KELT-1 system, we expect it to provide an important testbed for theories of the emplacement and evolution of short-period companions, and theories of tidal dissipation and irradiated brown dwarf atmospheres.

astro-ph.EP↗

arXiv2012

The Very Massive and Hot LMC Star VFTS 682: Progenitor of a Future Dark Gamma-Ray Burst?

VFTS 682, a very massive and very hot Wolf-Rayet (WR) star recently discovered in the Large Magellanic Cloud near the famous star cluster R136, might be providing us with a glimpse of a missing link in our understanding of Long Gamma-Ray Bursts (LGRBs), including dark GRBs. It is likely its properties result from chemically homogeneous evolution (CHE), believed to be a key process for a massive star to become a GRB. It is also heavily obscured by dust extinction, which could make it a dark GRB upon explosion. Using Spitzer data we investigate the properties of interstellar dust in the vicinity of R136, and argue that its high obscuration is not unusual for its environment and that it could indeed be a slow runaway ("walkaway") from R136. Unfortunately, based on its current mass loss rate, VFTS 682 is unlikely to become a GRB, because it will lose too much angular momentum at its death. If it were to become a GRB, it probably would also not be dark, either escaping or destroying its surrounding dusty region. Nevertheless, it is a very interesting star, deserving further studies, and being one of only three presently identified WR stars (two others in the Small Magellanic Cloud) that seems to be undergoing CHE.

astro-ph.GA↗

arXiv2012

The Panchromatic Hubble Andromeda Treasury

The Panchromatic Hubble Andromeda Treasury (PHAT) is an on-going HST Multicycle Treasury program to image ~1/3 of M31's star forming disk in 6 filters, from the UV to the NIR. The full survey will resolve the galaxy into more than 100 million stars with projected radii from 0-20 kpc over a contiguous 0.5 square degree area in 828 orbits, producing imaging in the F275W and F336W filters with WFC3/UVIS, F475W and F814W with ACS/WFC, and F110W and F160W with WFC3/IR. The resulting wavelength coverage gives excellent constraints on stellar temperature, bolometric luminosity, and extinction for most spectral types. The photometry reaches SNR=4 at F275W=25.1, F336W=24.9, F475W=27.9, F814W=27.1, F110W=25.5, and F160W=24.6 for single pointings in the uncrowded outer disk; however, the optical and NIR data are crowding limited, and the deepest reliable magnitudes are up to 5 magnitudes brighter in the inner bulge. All pointings are dithered and produce Nyquist-sampled images in F475W, F814W, and F160W. We describe the observing strategy, photometry, astrometry, and data products, along with extensive tests of photometric stability, crowding errors, spatially-dependent photometric biases, and telescope pointing control. We report on initial fits to the structure of M31's disk, derived from the density of RGB stars, in a way that is independent of the assumed M/L and is robust to variations in dust extinction. These fits also show that the 10 kpc ring is not just a region of enhanced recent star formation, but is instead a dynamical structure containing a significant overdensity of stars with ages >1 Gyr. (Abridged)

astro-ph.CO↗

arXiv2012

The Unusual Temporal and Spectral Evolution of the Type IIn Supernova 2011ht

We present very early UV to optical photometric and spectroscopic observations of the peculiar Type IIn supernova (SN) 2011ht in UGC 5460. The UV observations of the rise to peak are only the second ever recorded for a Type IIn SN and are by far the most complete. The SN, first classified as a SN impostor, slowly rose to a peak of M_V \sim -17 in \sim55 days. In contrast to the \sim2 magnitude increase in the v-band light curve from the first observation until peak, the UV flux increased by >7 magnitudes. The optical spectra are dominated by strong, Balmer emission with narrow peaks (FWHM\sim600 km/s), very broad asymmetric wings (FWHM\sim4200 km/s), and blue shifted absorption (\sim300 km/s) superposed on a strong blue continuum. The UV spectra are dominated by FeII, MgII, SiII, and SiIII absorption lines broadened by \sim1500 km/s. Merged X-ray observations reveal a L_(0.2-10)=(1.0+/-0.2)x10^(39) erg/s. Some properties of SN 2011ht are similar to SN impostors, while others are comparable to Type IIn SNe. Early spectra showed features typical of luminous blue variables at maximum and during giant eruptions. However, the broad emission profiles coupled with the strong UV flux have not been observed in previous SN impostors. The absolute magnitude and energetics (~2.5x10^(49) ergs in the first 112 days) are reminiscent of normal Type IIn SN, but the spectra are of a dense wind. We suggest that the mechanism for creating this unusual profile could be a shock interacting with a shell of material that was ejected a year before the discovery of the SN.

astro-ph.CO↗

arXiv2012

Dust to Dust: 3 years in the Evolution of the Unusual SN 2008S

We obtained late-time optical and near-IR imaging of SN 2008S with the Large Binocular Telescope (LBT), near-IR data with the Hubble Space Telescope (HST), and mid-IR data with the Spitzer Space Telescope (SST). We find that (1) it is again invisible at optical (UBVR) wavelengths to magnitude limits of approximately 25 mag, (2) while detected in the near-IR (H) at approximately 24.8 mag, it is fading rapidly, and (3) it is still brighter than the progenitor at 3.6 and 4.5 microns in the mid-IR with a slow, steady decline. The IR detections in December 2010 are consistent with dust emission at a blackbody temperature of T ~ 640 K and a total luminosity of L ~ 200000 Lsun, much higher than the L ~ 40000 Lsun luminosity of the obscured progenitor star. The local environment also shows no evidence for massive (M >= 10 Msun) stars in the vicinity of the transient, consistent with the progenitor being a massive AGB star.

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arXiv2012

Unmasking the Supernova Impostors

(ABRIDGED) The canonical picture of a supernova impostor is a -11 < M_V < -14 optical transient from a massive (M > 40Msun) star during which the star ejects a dense shell of material. Dust formed in the ejecta then obscures the star. In this picture, the geometric expansion of the shell leads to clear predictions for the evolution of the optical depths and hence the evolution of the optical through mid-IR emissions. Here we review the theory of this standard model and then examine the impostors SN1954J, SN1997bs, SN1999bw, SN2000ch, SN2001ac, SN2002bu, SN2002kg and SN2003gm, as well as the potential archetype eta Carinae. SN1999bw, SN2000ch, SN2001ac, SN2002bu and SN2003gm all show mid-IR emission indicative of dust, and the luminosities of SN1999bw, SN2001ac, SN2002bu and SN2003gm are dominated by dust emission. The properties of these sources are broadly inconsistent with the predictions of the canonical model. There are probably two classes of sources. In one class (eta Carinae, SN1954J, SN1997bs, and (maybe) SN2003gm), the optical transient is a signal that the star is entering a phase with very high mass loss rates that must last far longer than the visual transient. The second class (SN1999bw, SN2001ac, SN2002bu and (maybe) SN2003gm) has the different physics of SN2008S and the 2008 NGC300 transient, where they are obscured by dust re-forming in a pre-existing wind after it was destroyed by an explosive transient. There are no cases where the source at late times is significantly fainter than the progenitor star. All these dusty transients are occurring in relatively low mass (M < 25Msun) stars rather than high mass (M > 40Msun) stars radiating near the Eddington limit like eta Carinae. The durations and energetics of these transients cannot be properly characterized without near/mid-IR observations.

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arXiv2011

A Reverberation Lag for the High-Ionization Component of the Broad Line Region in the Narrow-Line Seyfert 1 Mrk 335

We present the first results from a detailed analysis of photometric and spectrophotometric data on the narrow-line Seyfert 1 galaxy Mrk 335, collected over a 120-day span in the fall of 2010. From these data we measure the lag in the He II 4686 broad emission line relative to the optical continuum to be 2.7 \pm 0.6 days and the lag in the Hβ4861 broad emission line to be 13.9 \pm 0.9 days. Combined with the line width, the He II lag yields a black hole mass, MBH = (2.6 \pm 0.8)\times 10^7 Msun. This measurement is consistent with measurements made using the Hβ4861 line, suggesting that the He II emission originates in the same region as Hβ, but at a much smaller radius. This constitutes the first robust lag measurement for a high-ionization line in a narrow-line Seyfert 1 galaxy.

astro-ph.CO↗

arXiv2011

Discovery of Variability of the Progenitor of SN 2011dh in M51 Using the Large Binocular Telescope

We show that the candidate progenitor of the core-collapse SN 2011dh in M51 (8 Mpc away) was fading by 0.039 +- 0.006 mag/year during the three years prior to the supernova, and that this level of variability is moderately unusual for other similar stars in M 51. While there are uncertainties about whether the true progenitor was a blue companion to this candidate, the result illustrates that there are no technical challenges to obtaining fairly high precision light curves of supernova progenitors using ground based observations of nearby (<10 Mpc) galaxies with wide field cameras on 8m-class telescopes. While other sources of variability may dominate, it is even possible to reach into the range of evolution rates required by the quasi-static evolution of the stellar envelope. For M 81, where we have many more epochs and a slightly longer time baseline, our formal 3 sigma sensitivity to slow changes is presently 3 millimag/year for a M_V ~= -8 mag star. In short, there is no observational barrier to determining whether the variability properties of stars in their last phases of evolution (post Carbon ignition) are different from earlier phases.

astro-ph.CO↗

arXiv2011

SN 2008jb: A "Lost" Core-Collapse Supernova in a Star-Forming Dwarf Galaxy at ~10 Mpc

We present the discovery and follow-up observations of SN 2008jb, a core-collapse supernova in the dwarf irregular galaxy ESO 302-14 at 9.6 Mpc. This transient was missed by galaxy-targeted surveys and was only found in archival optical images obtained by CRTS and ASAS. It was detected shortly after explosion and reached a bright optical maximum, Vmax = 13.6 mag (M_Vmax ~ -16.5). The shape of the light curve shows a plateau of 100 days, followed by a drop of 1.4 mag in V-band to a decline with the approximate Co 56 decay slope, consistent with 0.04 Msun of Ni 56 synthesized in the explosion. A spectrum obtained 2 years after explosion shows a broad, boxy Halpha emission line, which is unusual for type IIP supernovae. We detect the supernova in archival Spitzer and WISE images obtained 8-14 months after explosion, which show clear signs of warm dust emission. The dwarf irregular host galaxy has a low gas-phase oxygen abundance, 12 + log(O/H) = 8.2 (~1/5 Solar), similar to those of the SMC and the hosts of long gamma-ray bursts and luminous core-collapse supernovae. We study the host environment using GALEX FUV, R-band, and Halpha images and find that the supernova occurred in a large star-formation complex. The morphology of the Halpha emission appears as a large shell (R = 350 pc) surrounding the FUV and optical emission. We estimate an age of ~9 Myr and a total mass of ~2 x 10^5 Msun for the star-formation complex. These properties are consistent with the expanding Halpha supershells observed in well-studied nearby dwarf galaxies, which are tell-tale signs of feedback from the cumulative effect of massive star winds and supernovae. The age estimated for the star-forming region suggests a relatively high-mass progenitor star with initial mass of ~20 Msun. We discuss the implications of these findings in the study of core-collapse supernova progenitors. (Abridged)

astro-ph.SR↗

arXiv2011

The Cosmic Core-collapse Supernova Rate does not match the Massive-Star Formation Rate

We identify a "supernova rate problem": the measured cosmic core-collapse supernova rate is a factor of ~ 2 smaller (with significance ~ 2 sigma) than that predicted from the measured cosmic massive-star formation rate. The comparison is critical for topics from galaxy evolution and enrichment to the abundance of neutron stars and black holes. We systematically explore possible resolutions. The accuracy and precision of the star formation rate data and conversion to the supernova rate are well supported, and proposed changes would have far-reaching consequences. The dominant effect is likely that many supernovae are missed because they are either optically dim (low-luminosity) or dark, whether intrinsically or due to obscuration. We investigate supernovae too dim to have been discovered in cosmic surveys by a detailed study of all supernova discoveries in the local volume. If possible supernova impostors are included, then dim supernovae are common enough by fraction to solve the supernova rate problem. If they are not included, then the rate of dark core collapses is likely substantial. Other alternatives are that there are surprising changes in our understanding of star formation or supernova rates, including that supernovae form differently in small galaxies than in normal galaxies. These possibilities can be distinguished by upcoming supernova surveys, star formation measurements, searches for disappearing massive stars, and measurements of supernova neutrinos.

astro-ph.CO↗