- ID:
- ivo://CDS.VizieR/J/AJ/135/966
- Title:
- Circumstellar disk evolution in NGC 2068/71
- Short Name:
- J/AJ/135/966
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We study the disk and accretion properties of young stars in the NGC 2068 and NGC 2071 clusters. Using low-resolution optical spectra, we define a membership sample and determine an age for the region of ~2Myr. Using high-resolution spectra of the H{alpha} line we study the accretion activity of these likely members and also examine the disk properties of the likely members using IRAC and MIPS mid-infrared photometry. A substantial fraction (79%) of the 67 members have an infrared excess while all of the stars with significant infrared excess show evidence for active accretion. We find three populations of evolved disks (IRAC weak, MIPS weak, and transition disks) all of which show decreased accretion activity in addition to the evidence for evolution in the dust disk.
Number of results to display per page
Search Results
- ID:
- ivo://CDS.VizieR/J/A+A/293/833
- Title:
- Circumstellar silicate grains
- Short Name:
- J/A+A/293/833
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Based on the radiative transfer modelling studies relative to a large sample of circumstellar envelopes (~300 sources) undertaken by David & Papoular (1990A&A...237..425D) the complex dielectric function for circumstellar silicate grains is derived in the wavelength region from 0.35 to ~1355m.
- ID:
- ivo://CDS.VizieR/J/ApJ/666/757
- Title:
- CIV 1549{AA} emission line in AGNs
- Short Name:
- J/ApJ/666/757
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We are exploring a spectroscopic unification for all types of broad-line emitting AGNs. The four-dimensional Eigenvector 1 (4DE1) parameter space organizes quasar diversity in a sequence primarily governed by Eddington ratio. This paper considers the role of CIV {lambda}1549 measures as 4DE1 diagnostics. We use HST archival spectra for 130 sources with S/N high enough to permit reliable CIV {lambda}1549 broad-component measures.
- ID:
- ivo://CDS.VizieR/J/A+A/473/791
- Title:
- CIV absorption in DLAs and sub-DLAs systems
- Short Name:
- J/A+A/473/791
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a study of CIV absorption in a sample of 63 damped Lyman-{alpha} (DLA) systems and 11 sub-DLAs in the redshift range 1.75<z_abs_<3.61, using a dataset of high-resolution (6.6km/s FWHM), high signal-to-noise VLT/UVES spectra. The complex absorption line profiles show both narrow and broad CIV components, indicating the presence of both warm, photoionized and hot, collisionally ionized gas. We report new correlations between the metallicity (measured in the neutral-phase) and each of the CIV column density, the CIV total line width, and the maximum CIV velocity.
- ID:
- ivo://CDS.VizieR/J/ApJ/702/767
- Title:
- CIV and FeK{alpha} Baldwin effects in AGNs
- Short Name:
- J/ApJ/702/767
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We use UV/optical and X-ray observations of 272 radio-quiet Type 1 active galactic nuclei and quasars to investigate the CIV Baldwin Effect (BEff). The UV/optical spectra are drawn from the Hubble Space Telescope, International Ultraviolet Explorer and Sloan Digital Sky Survey archives. The X-ray spectra are from the Chandra and XMM-Newton archives. We apply correlation and partial-correlation analyses to the equivalent widths (EWs), continuum monochromatic luminosities, and {alpha}_ox_, which characterizes the relative X-ray to UV brightness. The EW of the CIV{lambda}1549 emission line is correlated with both {alpha}_ox_ and luminosity. In our sample, narrow Fe K{alpha} 6.4keV emission lines are detected in 50 objects.
- ID:
- ivo://CDS.VizieR/J/ApJ/777/168
- Title:
- CIV and SiIV broad absorption line troughs in SDSS
- Short Name:
- J/ApJ/777/168
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We present a detailed investigation of the variability of 428 CIV and 235 SiIV broad absorption line (BAL) troughs identified in multi-epoch observations of 291 quasars by the Sloan Digital Sky Survey-I/II/III. These observations primarily sample rest-frame timescales of 1-3.7yr over which significant rearrangement of the BAL wind is expected. We derive a number of observational results on, e.g., the frequency of BAL variability, the velocity range over which BAL variability occurs, the primary observed form of BAL-trough variability, the dependence of BAL variability upon timescale, the frequency of BAL strengthening versus weakening, correlations between BAL variability and BAL-trough profiles, relations between CIV and SiIV BAL variability, coordinated multi-trough variability, and BAL variations as a function of quasar properties. We assess implications of these observational results for quasar winds. Our results support models where most BAL absorption is formed within an order-of-magnitude of the wind-launching radius, although a significant minority of BAL troughs may arise on larger scales. We estimate an average lifetime for a BAL trough along our line-of-sight of a few thousand years. BAL disappearance and emergence events appear to be extremes of general BAL variability, rather than being qualitatively distinct phenomena. We derive the parameters of a random-walk model for BAL EW variability, finding that this model can acceptably describe some key aspects of EW variability. The coordinated trough variability of BAL quasars with multiple troughs suggests that changes in "shielding gas" may play a significant role in driving general BAL variability.
- ID:
- ivo://CDS.VizieR/J/A+A/616/A114
- Title:
- CIV BAL disappearance in SDSS QSOs
- Short Name:
- J/A+A/616/A114
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- Broad absorption lines (BALs) in the spectra of quasi-stellar objects (QSOs) originate from outflowing winds along our line of sight; winds are thought to originate from the inner regions of the QSO accretion disk, close to the central supermassive black hole (SMBH). These winds likely play a role in galaxy evolution and are responsible for aiding the accretion mechanism onto the SMBH. Several works have shown that BAL equivalent widths can change on typical timescales from months to years; such variability is generally attributed to changes in the covering factor (due to rotation and/or changes in the wind structure) and/or in the ionization level of the gas. We investigate BAL variability, focusing on BAL disappearance. We analyze multi-epoch spectra of more than 1500 QSOs - the largest sample ever used for such a study - observed by different programs from the Sloan Digital Sky Survey-I/II/III (SDSS-I/II/III), and search for disappearing CIV BALs. The spectra cover a rest-frame time baseline ranging from 0.28 to 4.9yr; the source redshifts range from 1.68 to 4.27. We detect 73 disappearing BALs in the spectra of 67 unique sources. This corresponds to 3.9% of BALs disappearing within 4.9yr (rest frame), and 5.1% of our BAL QSOs exhibit at least one disappearing BAL within 4.9yr (rest frame). We estimate the average lifetime of a BAL along our line of sight (~=80-100yr), which appears consistent with the accretion disk orbital time at distances where winds are thought to originate. We inspect various properties of the disappearing BAL sample and compare them to the corresponding properties of our main sample. We also investigate the existence of a correlation in the variability of multiple troughs in the same spectrum, and find it persistent at large velocity offsets between BAL pairs, suggesting that a mechanism extending on a global scale is necessary to explain the phenomenon. We select a more reliable sample of disappearing BALs on the basis of some criteria adopted in a previous publication, where a subset of our current sample was analyzed, and compare the findings from the two works, obtaining generally consistent results.
- ID:
- ivo://CDS.VizieR/J/MNRAS/454/3962
- Title:
- 188 CIV BAL QSOs from SDSS DR7
- Short Name:
- J/MNRAS/454/3962
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- The variability of broad absorption lines (BALs) is investigated for a sample of 188 BAL quasars (QSOs) (z>1.7) with at least two epoch observations from the Sloan Digital Sky Survey Data Release 7, covering a time-scale of about 0.001-3yr in the rest frame. Considering only the longest time-scale between epochs for each QSO, 73 variable regions in the CIV BAL troughs are detected for 43 BAL QSOs. The proportion of BAL QSOs showing variable regions increases with longer time-interval than about 1 yr in the rest frame. The velocity width of variable regions is narrow compared to the BAL-trough outflow velocity. For 43 BAL QSOs with variable regions, it is found that there is a medium strong correlation between the variation of the continuum luminosity at 1500{AA} and the variation of the spectral index. With respect to the total 188 QSOs, a larger proportion of BAL QSOs with variable regions appears bluer during their brighter phases, which implies that the origin of BAL variable regions is related to the central accretion process. For 43 BAL QSOs with variable regions, it is possible that there is a negative medium strong correlation between the absolute variation of the equivalent width and the Mgii-based black hole mass, and a medium strong correlation between the maximum outflow velocity of variable regions and the Eddington ratio. These results imply a connection between the BAL-trough variation and the central accretion process.
- ID:
- ivo://CDS.VizieR/J/ApJ/791/88
- Title:
- C IV BAL troughs properties in quasars
- Short Name:
- J/ApJ/791/88
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We consider how the profile and multi-year variability properties of a large sample of C IV Broad Absorption Line (BAL) troughs change when BALs from Si IV and/or Al III are present at corresponding velocities, indicating that the line of sight intercepts at least some lower ionization gas. We derive a number of observational results for C IV BALs separated according to the presence or absence of accompanying lower ionization transitions, including measurements of composite profile shapes, equivalent width (EW), characteristic velocities, composite variation profiles, and EW variability. We also measure the correlations between EW and fractional-EW variability for C IV, Si IV, and Al III. Our measurements reveal the basic correlated changes between ionization level, kinematics, and column density expected in accretion-disk wind models; e.g., lines of sight including lower ionization material generally show deeper and broader C IV troughs that have smaller minimum velocities and that are less variable. Many C IV BALs with no accompanying Si IV or Al III BALs may have only mild or no saturation.
- ID:
- ivo://CDS.VizieR/J/ApJ/862/22
- Title:
- CIV broad abs. line SDSS QSO obs. with Gemini
- Short Name:
- J/ApJ/862/22
- Date:
- 21 Oct 2021
- Publisher:
- CDS
- Description:
- We isolate a set of quasars that exhibit emergent CIV broad absorption lines (BALs) in their spectra by comparing spectra in the Sloan Digital Sky Survey (SDSS) Data Release 7 and the SDSS/BOSS Data Releases 9 and 10. After visually defining a set of emergent BALs, follow-up observations were obtained with the Gemini Observatory for 105 quasars. We find an emergence rate consistent with the previously reported disappearance rate of BAL quasars given the relative numbers of non-BAL and BAL quasars in the SDSS. We find that candidate newly emerged BALs are preferentially drawn from among BALs with smaller balnicity indices, shallower depths, larger velocities, and smaller widths. Within two rest-frame years (average) after a BAL has emerged, we find it equally likely to continue increasing in equivalent width in an observation 6 months later (average) as it is to start decreasing. From the time separations between our observations, we conclude that the coherence timescale of BALs is less than 100 rest-frame days. We observe coordinated variability among pairs of troughs in the same quasar, likely due to clouds at different velocities responding to the same changes in ionizing flux, and the coordination is stronger if the velocity separation between the two troughs is smaller. We speculate that the latter effect may be due to clouds having on average lower densities at higher velocities owing to mass conservation in an accelerating flow, causing the absorbing gas in those clouds to respond on different timescales to the same ionizing flux variations.
