Sidney Wolff - Academia.edu (original) (raw)

Papers by Sidney Wolff

Sky and Telescope, Sep 1, 1988

Astrophys J, 1974

BD + 37 deg 1977, one of the hottest objects identified by observations with the ESRO TD 1A satel... more BD + 37 deg 1977, one of the hottest objects identified by observations with the ESRO TD 1A satellite, is shown to be a subluminous O-type star. Helium line profiles of the star are illustrated.

Astrophys J, 1974

... However, both these errors affect the magnitude of the field and not the sign, and should ten... more ... However, both these errors affect the magnitude of the field and not the sign, and should tend to cancel when averaged over a symmetrically reversing magnetic ... which has a spectrum generally similar to 49 Cam, and a value of v sin i less than 3 km s ` (Evans and Elste 1971). ...

Astronomy and Astrophysics, Nov 29, 1974

The Observatory, Feb 1, 1996

We report the results of a study aimed at comparing the distribution of stellar rotational veloci... more We report the results of a study aimed at comparing the distribution of stellar rotational velocities, N(vsini), derived from WIYN-Hydra spectroscopy of a sample of 200 B0-B9 star members of the young (t 10 Myr), high stellar density h and chi Perseii cluster with those obtained for a sample of field B stars having comparable age; this latter sample is presumably dominated by stars born in much lower density environments. We find that the median value of vsini among the h and chi Perseii sample significantly exceeds that for the field B-stars, and discuss this result in the context of models that posit (a) that stellar angular momenta are regulated by disk-locking, and (b) time averaged disk acretion rates are higher in regions of higher initial stellar density.

(Abridged) Projected rotational velocities (vsini) have been measured for a sample of 145 stars w... more (Abridged) Projected rotational velocities (vsini) have been measured for a sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun) located in the Orion star-forming complex. These measurements have been supplemented with data from the literature for Orion stars with masses as low as 0.1 Msun. The primary finding from analysis of these data is that the upper envelope of the observed values of angular momentum per unit mass (J/M) varies as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3 Msun. This power law extends smoothly into the domain of more massive stars (3 to 10 Msun), which in Orion are already on the ZAMS. This result stands in sharp contrast to the properties of main sequence stars, which show a break in the power law and a sharp decline in J/M with decreasing mass for stars with M <2 Msun. A second result of our study is that this break is seen already among the PMS stars in our Orion sample that are on radiative tracks, even though these stars are only a few million years old. A comparison of rotation rates seen for stars on either side of the convective-radiative boundary shows that stars do not rotate as solid bodies during the transition from convective to radiative tracks.

To investigate what happens to angular momentum during the earliest observable phases of stellar ... more To investigate what happens to angular momentum during the earliest observable phases of stellar evolution, we combined our measurements of periods (P), projected rotational velocities (v sin i), and supporting data on K5-M2 stars (corresponding to masses 0.25 to 1 M⊙ ) with those published in the literature for the Orion Nebula Cluster and environs, ρ Oph, TW Hydra, Taurus-Auriga, NGC 2264, Chamaeleon, Lupus, and η Cha. We combine these measures of rotation with the derived values of stellar R (as determined from L bol and T eff) to compare the data with two extreme scenarios for the evolution of PMS angular momenta: conservation of stellar angular velocity and conservation of stellar angular momentum. Both the P and v sin i datasets suggest that a significant fraction of all PMS stars must evolve at nearly constant angular velocity during the first ˜3-5 million years after they begin their evolution down convective tracks. Hence, the angular momenta of a significant fraction of PMS stars must be tightly regulated during the first few million years after they first become observable. This result seems surprising at first glance, because observations of young main sequence stars reveal a population (30-40%) of rapidly rotating stars that must begin to spin up at ages t << 5 Myr. To determine whether these apparently contradictory results are reconcilable, we make use of simple simulations and find that a modest population (30-40%) of PMS stars could in fact be released within the first 1 Myr and still produce period distributions statistically consistent with the observed data.

Astronomical Journal, 2005

Projected rotational velocities (vsini) have been measured for 216 B0-B9 stars in the rich, dense... more Projected rotational velocities (vsini) have been measured for 216 B0-B9 stars in the rich, dense h and χ Persei double cluster and compared with the distribution of rotational velocities for a sample of field stars having comparable ages (t~12-15 Myr) and masses (M~4-15 Msolar). For stars that are relatively little evolved from their initial locations on the zero-age main sequence (ZAMS) (those with masses M~4-5 Msolar), the mean vsini measured for the h and χ Per sample is slightly more than 2 times larger than the mean determined for field stars of comparable mass, and the cluster and field vsini distributions differ with a high degree of significance. For somewhat more evolved stars with masses in the range 5-9 Msolar, the mean vsini in h and χ Per is 1.5 times that of the field; the vsini distributions differ as well, but with a lower degree of statistical significance. For stars that have evolved significantly from the ZAMS and are approaching the hydrogen exhaustion phase (those with masses in the range 9-15 Msolar), the cluster and field star means and distributions are only slightly different. We argue that both the higher rotation rates and the pattern of rotation speeds as a function of mass that differentiate main-sequence B stars in h and χ Per from their field analogs were likely imprinted during the star formation process rather than a result of angular momentum evolution over the 12-15 Myr cluster lifetime. We speculate that these differences may reflect the effects of the higher accretion rates that theory suggests are characteristic of regions that give birth to dense clusters, namely, (1) higher initial rotation speeds; (2) higher initial radii along the stellar birth line, resulting in greater spin-up between the birth line and the ZAMS; and (3) a more pronounced maximum in the birth line radius-mass relationship that results in differentially greater spin-up for stars that become mid- to late-B stars on the ZAMS.

The smallest molecular cores observed to date have at least ˜6 orders of magnitude greater angula... more The smallest molecular cores observed to date have at least ˜6 orders of magnitude greater angular momentum per unit mass than the Sun, suggesting that they would greatly exceed the breakup velocity if no angular momentum was lost during the star formation process. Therefore, an angular momentum regulation mechanism must be at work in the pre-main-sequence phase, and disks are often invoked as the solution to the angular momentum problem. Thanks to large-format CCDs, more than 1000 periods for young stars are now known (with more being presented at this conference), and with the Spitzer Space Telescope, we have the ability to get reliable circumstellar disk indicators for many 1000s of stars at once. Now, for the first time, we may have enough stars to start to constrain the angular momentum loss mechanism in a meaningful fashion. In this contribution, we review the observations made to date of rotation in pre-main-sequence low-mass stars.

We have compiled data on rotational velocities for more than 1000 K and M stars in 12 young clust... more We have compiled data on rotational velocities for more than 1000 K and M stars in 12 young clusters ranging in age from Orion to the Hyades. These data enable a search for systematic changes in stellar rotational velocity vs. age. Taken together, these data show that most pre-main sequence (PMS) stars spanning ages from about 0.1 to about 1 Myr do not appear to spin up in response to contraction down their convective tracks, that spin up between 1 and 3 million years is modest at best, and that some stars do not appear to spin up at all. These results extend and reinforce our earlier study (Rebull et al. 2002), based on observations of several hundred stars in the Orion Flanking Fields, NGC 2264, and the Orion Nebula Cluster (ONC), which showed that the majority of PMS stars in these three groups apparently do not conserve stellar angular momentum as they contract, but instead evolve at nearly constant angular velocity. This result applies both to stars with and without near-IR I-K excesses indicative of disks.

Astrophys J, 1974

... EQUIVALENT W IDTHS (A) HD 153919 H + He It: He I: He ii: N III: SIDNEY C. WOLFF AND NANCY D. ... more ... EQUIVALENT W IDTHS (A) HD 153919 H + He It: He I: He ii: N III: SIDNEY C. WOLFF AND NANCY D. MORRISON Vol. ... polarization to magnetic field is possible only if the strength of the underlying absorption line, uncontaminated by emission, is known (eg, Angel, McGraw, and ...

We have used the KPNO 4m and the Keck I telescopes to obtain spectroscopic rotational velocities ... more We have used the KPNO 4m and the Keck I telescopes to obtain spectroscopic rotational velocities of M dwarf members of the Pleiades and Hyades in order to define better the angular momentum evolutionary history of low mass stars. The new data include 60 Pleiades members with 8.5 < M_V < 11.5, and 20 Hyades members with 12.6 < M_V < 15.6. Some empirical aspects of our analysis of these spectra include: a) fainter than M_V ~10.5, apparently all Pleiades members have rotational velocities above v_rot ~6 km s^{-1}, corresponding to rotational periods less than about 3 days. The range in rotational velocity at a given color decreases with decreasing mass, going from a factor of more than 15 for early K dwarfs to only a factor of 2 or 3 for late M dwarfs; b) in terms of their angular rotational velocities, the most rapidly rotating stars in the Pleiades are M dwarfs, with inferred periods ranging from ~4 to ~8 hours; c) even by the age of the Hyades, the very low mass M dwarfs still rotate relatively rapidly, with spectroscopic rotational velocities up to about 35 km s^{-1}, and hence inferred rotational periods as short as about 8 hours. The late M dwarfs appear to decrease their rotation rate by perhaps a factor of two between the age of the Pleiades and the age of the Hyades. We will discuss the implications of these observations using new rotational velocity evolution models for low mass stars constructed using an updated version of the Yale evolutionary code.

Sky and Telescope, Sep 1, 1988

Astrophys J, 1974

BD + 37 deg 1977, one of the hottest objects identified by observations with the ESRO TD 1A satel... more BD + 37 deg 1977, one of the hottest objects identified by observations with the ESRO TD 1A satellite, is shown to be a subluminous O-type star. Helium line profiles of the star are illustrated.

Astrophys J, 1974

... However, both these errors affect the magnitude of the field and not the sign, and should ten... more ... However, both these errors affect the magnitude of the field and not the sign, and should tend to cancel when averaged over a symmetrically reversing magnetic ... which has a spectrum generally similar to 49 Cam, and a value of v sin i less than 3 km s ` (Evans and Elste 1971). ...

Astronomy and Astrophysics, Nov 29, 1974

The Observatory, Feb 1, 1996

We report the results of a study aimed at comparing the distribution of stellar rotational veloci... more We report the results of a study aimed at comparing the distribution of stellar rotational velocities, N(vsini), derived from WIYN-Hydra spectroscopy of a sample of 200 B0-B9 star members of the young (t 10 Myr), high stellar density h and chi Perseii cluster with those obtained for a sample of field B stars having comparable age; this latter sample is presumably dominated by stars born in much lower density environments. We find that the median value of vsini among the h and chi Perseii sample significantly exceeds that for the field B-stars, and discuss this result in the context of models that posit (a) that stellar angular momenta are regulated by disk-locking, and (b) time averaged disk acretion rates are higher in regions of higher initial stellar density.

(Abridged) Projected rotational velocities (vsini) have been measured for a sample of 145 stars w... more (Abridged) Projected rotational velocities (vsini) have been measured for a sample of 145 stars with masses between 0.4 and >10 Msun (median mass 2.1 Msun) located in the Orion star-forming complex. These measurements have been supplemented with data from the literature for Orion stars with masses as low as 0.1 Msun. The primary finding from analysis of these data is that the upper envelope of the observed values of angular momentum per unit mass (J/M) varies as M^0.25 for stars on convective tracks having masses in the range ~0.1 to ~3 Msun. This power law extends smoothly into the domain of more massive stars (3 to 10 Msun), which in Orion are already on the ZAMS. This result stands in sharp contrast to the properties of main sequence stars, which show a break in the power law and a sharp decline in J/M with decreasing mass for stars with M <2 Msun. A second result of our study is that this break is seen already among the PMS stars in our Orion sample that are on radiative tracks, even though these stars are only a few million years old. A comparison of rotation rates seen for stars on either side of the convective-radiative boundary shows that stars do not rotate as solid bodies during the transition from convective to radiative tracks.

To investigate what happens to angular momentum during the earliest observable phases of stellar ... more To investigate what happens to angular momentum during the earliest observable phases of stellar evolution, we combined our measurements of periods (P), projected rotational velocities (v sin i), and supporting data on K5-M2 stars (corresponding to masses 0.25 to 1 M⊙ ) with those published in the literature for the Orion Nebula Cluster and environs, ρ Oph, TW Hydra, Taurus-Auriga, NGC 2264, Chamaeleon, Lupus, and η Cha. We combine these measures of rotation with the derived values of stellar R (as determined from L bol and T eff) to compare the data with two extreme scenarios for the evolution of PMS angular momenta: conservation of stellar angular velocity and conservation of stellar angular momentum. Both the P and v sin i datasets suggest that a significant fraction of all PMS stars must evolve at nearly constant angular velocity during the first ˜3-5 million years after they begin their evolution down convective tracks. Hence, the angular momenta of a significant fraction of PMS stars must be tightly regulated during the first few million years after they first become observable. This result seems surprising at first glance, because observations of young main sequence stars reveal a population (30-40%) of rapidly rotating stars that must begin to spin up at ages t << 5 Myr. To determine whether these apparently contradictory results are reconcilable, we make use of simple simulations and find that a modest population (30-40%) of PMS stars could in fact be released within the first 1 Myr and still produce period distributions statistically consistent with the observed data.

Astronomical Journal, 2005

Projected rotational velocities (vsini) have been measured for 216 B0-B9 stars in the rich, dense... more Projected rotational velocities (vsini) have been measured for 216 B0-B9 stars in the rich, dense h and χ Persei double cluster and compared with the distribution of rotational velocities for a sample of field stars having comparable ages (t~12-15 Myr) and masses (M~4-15 Msolar). For stars that are relatively little evolved from their initial locations on the zero-age main sequence (ZAMS) (those with masses M~4-5 Msolar), the mean vsini measured for the h and χ Per sample is slightly more than 2 times larger than the mean determined for field stars of comparable mass, and the cluster and field vsini distributions differ with a high degree of significance. For somewhat more evolved stars with masses in the range 5-9 Msolar, the mean vsini in h and χ Per is 1.5 times that of the field; the vsini distributions differ as well, but with a lower degree of statistical significance. For stars that have evolved significantly from the ZAMS and are approaching the hydrogen exhaustion phase (those with masses in the range 9-15 Msolar), the cluster and field star means and distributions are only slightly different. We argue that both the higher rotation rates and the pattern of rotation speeds as a function of mass that differentiate main-sequence B stars in h and χ Per from their field analogs were likely imprinted during the star formation process rather than a result of angular momentum evolution over the 12-15 Myr cluster lifetime. We speculate that these differences may reflect the effects of the higher accretion rates that theory suggests are characteristic of regions that give birth to dense clusters, namely, (1) higher initial rotation speeds; (2) higher initial radii along the stellar birth line, resulting in greater spin-up between the birth line and the ZAMS; and (3) a more pronounced maximum in the birth line radius-mass relationship that results in differentially greater spin-up for stars that become mid- to late-B stars on the ZAMS.

The smallest molecular cores observed to date have at least ˜6 orders of magnitude greater angula... more The smallest molecular cores observed to date have at least ˜6 orders of magnitude greater angular momentum per unit mass than the Sun, suggesting that they would greatly exceed the breakup velocity if no angular momentum was lost during the star formation process. Therefore, an angular momentum regulation mechanism must be at work in the pre-main-sequence phase, and disks are often invoked as the solution to the angular momentum problem. Thanks to large-format CCDs, more than 1000 periods for young stars are now known (with more being presented at this conference), and with the Spitzer Space Telescope, we have the ability to get reliable circumstellar disk indicators for many 1000s of stars at once. Now, for the first time, we may have enough stars to start to constrain the angular momentum loss mechanism in a meaningful fashion. In this contribution, we review the observations made to date of rotation in pre-main-sequence low-mass stars.

We have compiled data on rotational velocities for more than 1000 K and M stars in 12 young clust... more We have compiled data on rotational velocities for more than 1000 K and M stars in 12 young clusters ranging in age from Orion to the Hyades. These data enable a search for systematic changes in stellar rotational velocity vs. age. Taken together, these data show that most pre-main sequence (PMS) stars spanning ages from about 0.1 to about 1 Myr do not appear to spin up in response to contraction down their convective tracks, that spin up between 1 and 3 million years is modest at best, and that some stars do not appear to spin up at all. These results extend and reinforce our earlier study (Rebull et al. 2002), based on observations of several hundred stars in the Orion Flanking Fields, NGC 2264, and the Orion Nebula Cluster (ONC), which showed that the majority of PMS stars in these three groups apparently do not conserve stellar angular momentum as they contract, but instead evolve at nearly constant angular velocity. This result applies both to stars with and without near-IR I-K excesses indicative of disks.

Astrophys J, 1974

... EQUIVALENT W IDTHS (A) HD 153919 H + He It: He I: He ii: N III: SIDNEY C. WOLFF AND NANCY D. ... more ... EQUIVALENT W IDTHS (A) HD 153919 H + He It: He I: He ii: N III: SIDNEY C. WOLFF AND NANCY D. MORRISON Vol. ... polarization to magnetic field is possible only if the strength of the underlying absorption line, uncontaminated by emission, is known (eg, Angel, McGraw, and ...

We have used the KPNO 4m and the Keck I telescopes to obtain spectroscopic rotational velocities ... more We have used the KPNO 4m and the Keck I telescopes to obtain spectroscopic rotational velocities of M dwarf members of the Pleiades and Hyades in order to define better the angular momentum evolutionary history of low mass stars. The new data include 60 Pleiades members with 8.5 < M_V < 11.5, and 20 Hyades members with 12.6 < M_V < 15.6. Some empirical aspects of our analysis of these spectra include: a) fainter than M_V ~10.5, apparently all Pleiades members have rotational velocities above v_rot ~6 km s^{-1}, corresponding to rotational periods less than about 3 days. The range in rotational velocity at a given color decreases with decreasing mass, going from a factor of more than 15 for early K dwarfs to only a factor of 2 or 3 for late M dwarfs; b) in terms of their angular rotational velocities, the most rapidly rotating stars in the Pleiades are M dwarfs, with inferred periods ranging from ~4 to ~8 hours; c) even by the age of the Hyades, the very low mass M dwarfs still rotate relatively rapidly, with spectroscopic rotational velocities up to about 35 km s^{-1}, and hence inferred rotational periods as short as about 8 hours. The late M dwarfs appear to decrease their rotation rate by perhaps a factor of two between the age of the Pleiades and the age of the Hyades. We will discuss the implications of these observations using new rotational velocity evolution models for low mass stars constructed using an updated version of the Yale evolutionary code.