| 1. |
- Cuong Nguyen, Duy, et al.
(författare)
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Disk-Braking in Young Stars : Probing Rotation in Chamaeleon I and Taurus-Auriga
- 2009
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Annan publikation (populärvet., debatt m.m.)abstract
- We present a comprehensive study of rotation, disk and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of v sin i. The rotational velocities in both regions show a clear mass dependence, with F--K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M^0.5 for our sample which spans a mass range of ~0.16 to ~3 M_sun. This power law complements previous studies in Orion which estimated j is proportional to M^0.25 for < ~2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M_sun. For a subsample of 67 objects with mid-IR photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks.
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| 2. |
- Cuong Nguyen, Duy, et al.
(författare)
-
How Variable is Accretion in Young Stars?
- 2009
-
Annan publikation (populärvet., debatt m.m.)abstract
- We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Tau-Aur and Cha I. For all stars, we obtain typically four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the H-alpha 10% width and the CaII-8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the CaII fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while H-alpha 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that CaII fluxes are a more robust quantitative indicator of accretion than H-alpha 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates towards later spectral types, reflecting the dM/dt vs. M relationship. There is a gap between accretors and non-accretors, pointing to a rapid shutdown of accretion. We conclude that the ~2 orders of magnitude scatter in the dM/dt vs. M relationship is dominated by object-to-object scatter instead of intrinsic source variability.
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| 3. |
- Nguyen, Duy Cuong, et al.
(författare)
-
Disk Braking in young Stars : Probing Rotation in Chamaeleon i and Taurus-Auriga
- 2009
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 695, s. 1648-1656
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Tidskriftsartikel (refereegranskat)abstract
- We present a comprehensive study of rotation, disk, and accretion signatures for 144 T Tauri stars in the young (~2 Myr old) Chamaeleon I and Taurus-Auriga star-forming regions based on multi-epoch high-resolution optical spectra from the Magellan Clay 6.5 m telescope supplemented by mid-infrared photometry from the Spitzer Space Telescope. In contrast to previous studies in the Orion Nebula Cluster and NGC 2264, we do not see a clear signature of disk braking in Tau-Aur and Cha I. We find that both accretors and non-accretors have similar distributions of vsin i. This result could be due to different initial conditions, insufficient time for disk braking, or a significant age spread within the regions. The rotational velocities in both regions show a clear mass dependence, with F-K stars rotating on average about twice as fast as M stars, consistent with results reported for other clusters of similar age. Similarly, we find the upper envelope of the observed values of specific angular momentum j varies as M 0.5 for our sample which spans a mass range of ~0.16-3 M sun. This power law complements previous studies in Orion which estimated j vprop M 0.25 for lsim2 Myr stars in the same mass regime, and a sharp decline in j with decreasing mass for older stars (~10 Myr) with M < 2 M sun. Furthermore, the overall specific angular momentum of this ~10 Myr population is five times lower than that of non-accretors in our sample, and implies a stellar braking mechanism other than disk braking could be at work. For a subsample of 67 objects with mid-infrared photometry, we examine the connection between accretion signatures and dusty disks: in the vast majority of cases (63/67), the two properties correlate well, which suggests that the timescale of gas accretion is similar to the lifetime of inner disks.
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| 4. |
- Nguyen, Duy Cuong, et al.
(författare)
-
How Variable is Accretion in Young Stars?
- 2009
-
Ingår i: Astrophysical Journal. - 0004-637X .- 1538-4357. ; 694, s. L153-L157
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Tidskriftsartikel (refereegranskat)abstract
- We analyze the variability in accretion-related emission lines for 40 Classical T Tauri stars to probe the extent of accretion variations in young stellar objects. Our analysis is based on multi-epoch high-resolution spectra for young stars in Taurus-Auriga and Chamaeleon I. For all stars, we typically obtain four spectra, covering timescales from hours to months. As proxies for the accretion rate, we use the Hα 10% width and the Ca II-λ8662 line flux. We find that while the two quantities are correlated, their variability amplitude is not. Converted to accretion rates, the Ca II fluxes indicate typical accretion rate changes of 0.35 dex, with 32% exceeding 0.5 dex, while Hα 10% width suggests changes of 0.65 dex, with 66% exceeding 0.5 dex. We conclude that Ca II fluxes are a more robust quantitative indicator of accretion than Hα 10% width, and that intrinsic accretion rate changes typically do not exceed 0.5 dex on timescales of days to months. The maximum extent of the variability is reached after a few days, suggesting that rotation is the dominant cause of variability. We see a decline of the inferred accretion rates toward later spectral types, reflecting the \dot{M} versus M relationship. There is a gap between accretors and nonaccretors, pointing to a rapid shutdown of accretion. We conclude that the ~two orders of magnitude scatter in the \dot{M} versus M relationship is dominated by object-to-object scatter instead of intrinsic source variability.
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| 5. |
- Nguyen, Duy Cuong, et al.
(författare)
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Multiplicity in Star Formation : Close Binaries in Chamaeleon I and Taurus-Auriga
- 2009
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Konferensbidrag (övrigt vetenskapligt/konstnärligt)abstract
- A large fraction of stars are members of binary systems. Yet, the formation and early evolution of binary and multiple stars is not well understood theoretically and poorly constrained observationally. We present findings from a comprehensive spectroscopic survey for close binaries among nearly 200 members of the Chamaeleon I and Taurus-Auriga star-forming regions. We find that higher mass stars in our sample have a higher spectroscopic binary frequency than their lower mass counterparts, similar to what is seen in the field and for resolved (wide) binaries in young associations. More intriguingly, our observations also reveal a significantly larger fraction of tight binaries in Tau-Aur compared to Cha I, both for F-K stars and for M dwarfs, implying possible regional variations in the formation of multiple stars.
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